diff --git a/backend/src/main/java/de/svencarstensen/muh/domain/Sample.java b/backend/src/main/java/de/svencarstensen/muh/domain/Sample.java
index ee98772..205340b 100644
--- a/backend/src/main/java/de/svencarstensen/muh/domain/Sample.java
+++ b/backend/src/main/java/de/svencarstensen/muh/domain/Sample.java
@@ -27,6 +27,7 @@ public record Sample(
         LocalDateTime createdAt,
         LocalDateTime updatedAt,
         LocalDateTime completedAt,
+        String ownerAccountId,
         String createdByUserCode,
         String createdByDisplayName
 ) {
diff --git a/backend/src/main/java/de/svencarstensen/muh/security/AuthTokenService.java b/backend/src/main/java/de/svencarstensen/muh/security/AuthTokenService.java
index faccb1a..a658ca0 100644
--- a/backend/src/main/java/de/svencarstensen/muh/security/AuthTokenService.java
+++ b/backend/src/main/java/de/svencarstensen/muh/security/AuthTokenService.java
@@ -22,6 +22,9 @@ public class AuthTokenService {
             @Value("${muh.security.token-secret}") String secret,
             @Value("${muh.security.token-validity-hours:12}") long validityHours
     ) {
+        if (secret == null || secret.isBlank()) {
+            throw new IllegalStateException("MUH_TOKEN_SECRET muss gesetzt sein");
+        }
         this.secret = secret.getBytes(StandardCharsets.UTF_8);
         this.validitySeconds = Math.max(1, validityHours) * 3600L;
     }
diff --git a/backend/src/main/java/de/svencarstensen/muh/security/AuthorizationService.java b/backend/src/main/java/de/svencarstensen/muh/security/AuthorizationService.java
new file mode 100644
index 0000000..d590be1
--- /dev/null
+++ b/backend/src/main/java/de/svencarstensen/muh/security/AuthorizationService.java
@@ -0,0 +1,52 @@
+package de.svencarstensen.muh.security;
+
+import de.svencarstensen.muh.domain.AppUser;
+import de.svencarstensen.muh.domain.UserRole;
+import de.svencarstensen.muh.repository.AppUserRepository;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Service;
+import org.springframework.web.server.ResponseStatusException;
+
+@Service
+public class AuthorizationService {
+
+    private final AppUserRepository appUserRepository;
+
+    public AuthorizationService(AppUserRepository appUserRepository) {
+        this.appUserRepository = appUserRepository;
+    }
+
+    public AppUser requireActiveUser(String actorId, String message) {
+        return appUserRepository.findById(requireText(actorId, message))
+                .filter(AppUser::active)
+                .orElseThrow(() -> new ResponseStatusException(HttpStatus.FORBIDDEN, message));
+    }
+
+    public void requireAdmin(String actorId, String message) {
+        AppUser actor = requireActiveUser(actorId, message);
+        if (!isAdmin(actor)) {
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN, message);
+        }
+    }
+
+    public boolean isAdmin(AppUser user) {
+        return user.role() == UserRole.ADMIN;
+    }
+
+    public String accountId(AppUser user) {
+        if (user.accountId() == null || user.accountId().isBlank()) {
+            if (user.id() == null || user.id().isBlank()) {
+                throw new ResponseStatusException(HttpStatus.FORBIDDEN, "Benutzerkonto ungueltig");
+            }
+            return user.id().trim();
+        }
+        return user.accountId().trim();
+    }
+
+    private String requireText(String value, String message) {
+        if (value == null || value.isBlank()) {
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, message);
+        }
+        return value.trim();
+    }
+}
diff --git a/backend/src/main/java/de/svencarstensen/muh/service/CatalogService.java b/backend/src/main/java/de/svencarstensen/muh/service/CatalogService.java
index 97a1d27..c7ab3cb 100644
--- a/backend/src/main/java/de/svencarstensen/muh/service/CatalogService.java
+++ b/backend/src/main/java/de/svencarstensen/muh/service/CatalogService.java
@@ -14,6 +14,7 @@ import de.svencarstensen.muh.repository.FarmerRepository;
 import de.svencarstensen.muh.repository.MedicationCatalogRepository;
 import de.svencarstensen.muh.repository.PathogenCatalogRepository;
 import de.svencarstensen.muh.security.AuthTokenService;
+import de.svencarstensen.muh.security.AuthorizationService;
 import org.springframework.http.HttpStatus;
 import org.springframework.lang.NonNull;
 import org.springframework.data.mongodb.core.MongoTemplate;
@@ -66,6 +67,7 @@ public class CatalogService {
     private final AppUserRepository appUserRepository;
     private final MongoTemplate mongoTemplate;
     private final AuthTokenService authTokenService;
+    private final AuthorizationService authorizationService;
     private final BCryptPasswordEncoder passwordEncoder = new BCryptPasswordEncoder();
 
     public CatalogService(
@@ -75,7 +77,8 @@ public class CatalogService {
             AntibioticCatalogRepository antibioticRepository,
             AppUserRepository appUserRepository,
             MongoTemplate mongoTemplate,
-            AuthTokenService authTokenService
+            AuthTokenService authTokenService,
+            AuthorizationService authorizationService
     ) {
         this.farmerRepository = farmerRepository;
         this.medicationRepository = medicationRepository;
@@ -84,6 +87,7 @@ public class CatalogService {
         this.appUserRepository = appUserRepository;
         this.mongoTemplate = mongoTemplate;
         this.authTokenService = authTokenService;
+        this.authorizationService = authorizationService;
     }
 
     public ActiveCatalogSummary activeCatalogSummary() {
@@ -96,7 +100,8 @@ public class CatalogService {
         );
     }
 
-    public AdministrationOverview administrationOverview() {
+    public AdministrationOverview administrationOverview(String actorId) {
+        authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
         return new AdministrationOverview(listFarmerRows(), listMedicationRows(), listPathogenRows(), listAntibioticRows());
     }
 
@@ -128,7 +133,8 @@ public class CatalogService {
                 .toList();
     }
 
-    public List<FarmerRow> saveFarmers(List<FarmerMutation> mutations) {
+    public List<FarmerRow> saveFarmers(String actorId, List<FarmerMutation> mutations) {
+        authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
         for (FarmerMutation mutation : mutations) {
             if (isBlank(mutation.name())) {
                 continue;
@@ -191,7 +197,8 @@ public class CatalogService {
         return listFarmerRows();
     }
 
-    public List<MedicationRow> saveMedications(List<MedicationMutation> mutations) {
+    public List<MedicationRow> saveMedications(String actorId, List<MedicationMutation> mutations) {
+        authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
         for (MedicationMutation mutation : mutations) {
             if (isBlank(mutation.name()) || mutation.category() == null) {
                 continue;
@@ -254,7 +261,8 @@ public class CatalogService {
         return listMedicationRows();
     }
 
-    public List<PathogenRow> savePathogens(List<PathogenMutation> mutations) {
+    public List<PathogenRow> savePathogens(String actorId, List<PathogenMutation> mutations) {
+        authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
         for (PathogenMutation mutation : mutations) {
             if (isBlank(mutation.name()) || mutation.kind() == null) {
                 continue;
@@ -322,7 +330,8 @@ public class CatalogService {
         return listPathogenRows();
     }
 
-    public List<AntibioticRow> saveAntibiotics(List<AntibioticMutation> mutations) {
+    public List<AntibioticRow> saveAntibiotics(String actorId, List<AntibioticMutation> mutations) {
+        authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
         for (AntibioticMutation mutation : mutations) {
             if (isBlank(mutation.name())) {
                 continue;
diff --git a/backend/src/main/java/de/svencarstensen/muh/service/PortalService.java b/backend/src/main/java/de/svencarstensen/muh/service/PortalService.java
index 4f32846..19f08b9 100644
--- a/backend/src/main/java/de/svencarstensen/muh/service/PortalService.java
+++ b/backend/src/main/java/de/svencarstensen/muh/service/PortalService.java
@@ -36,20 +36,20 @@ public class PortalService {
 
         List<PortalSampleRow> sampleRows;
         if (sampleNumber != null) {
-            sampleRows = List.of(toPortalRow(sampleService.getSampleByNumber(sampleNumber)));
+            sampleRows = List.of(toPortalRow(sampleService.getSampleByNumber(actorId, sampleNumber)));
         } else if (farmerBusinessKey != null && !farmerBusinessKey.isBlank()) {
-            sampleRows = sampleService.samplesByFarmerBusinessKey(farmerBusinessKey).stream()
+            sampleRows = sampleService.samplesByFarmerBusinessKey(actorId, farmerBusinessKey).stream()
                     .filter(sample -> cowQuery == null || cowQuery.isBlank() || cowMatches(sample, cowQuery))
                     .map(this::toPortalRow)
                     .sorted(Comparator.comparing(PortalSampleRow::createdAt).reversed())
                     .toList();
         } else if (date != null) {
-            sampleRows = sampleService.samplesByDate(date).stream()
+            sampleRows = sampleService.samplesByDate(actorId, date).stream()
                     .map(this::toPortalRow)
                     .sorted(Comparator.comparing(PortalSampleRow::completedAt, Comparator.nullsLast(Comparator.reverseOrder())))
                     .toList();
         } else {
-            sampleRows = sampleService.completedSamples().stream()
+            sampleRows = sampleService.completedSamples(actorId).stream()
                     .limit(25)
                     .map(this::toPortalRow)
                     .toList();
@@ -58,13 +58,13 @@ public class PortalService {
         return new PortalSnapshot(
                 matchingFarmers,
                 sampleRows,
-                reportService.reportCandidates(),
+                reportService.reportCandidates(actorId),
                 includeUsers ? catalogService.listUsers(actorId) : List.of()
         );
     }
 
-    public List<PortalSampleRow> searchSamplesByCreatedDate(LocalDate date) {
-        return sampleService.samplesByCreatedDate(date).stream()
+    public List<PortalSampleRow> searchSamplesByCreatedDate(String actorId, LocalDate date) {
+        return sampleService.samplesByCreatedDate(actorId, date).stream()
                 .map(this::toPortalRow)
                 .sorted(Comparator.comparing(PortalSampleRow::createdAt).reversed())
                 .toList();
diff --git a/backend/src/main/java/de/svencarstensen/muh/service/ReportService.java b/backend/src/main/java/de/svencarstensen/muh/service/ReportService.java
index 1bf5e0d..61277fc 100644
--- a/backend/src/main/java/de/svencarstensen/muh/service/ReportService.java
+++ b/backend/src/main/java/de/svencarstensen/muh/service/ReportService.java
@@ -53,8 +53,8 @@ public class ReportService {
         this.reportMailTemplate = loadTemplate(reportMailTemplateResource);
     }
 
-    public List<ReportCandidate> reportCandidates() {
-        return sampleService.completedSamples().stream()
+    public List<ReportCandidate> reportCandidates(String actorId) {
+        return sampleService.completedSamples(actorId).stream()
                 .filter(sample -> sample.farmerEmail() != null && !sample.farmerEmail().isBlank())
                 .map(this::toCandidate)
                 .toList();
@@ -66,7 +66,7 @@ public class ReportService {
         List<ReportCandidate> skipped = new ArrayList<>();
 
         for (String sampleId : sampleIds) {
-            Sample sample = sampleService.loadSampleEntity(sampleId);
+            Sample sample = sampleService.loadSampleEntity(actorId, sampleId);
             if (sample.farmerEmail() == null || sample.farmerEmail().isBlank() || sample.reportBlocked()) {
                 skipped.add(toCandidate(sample));
                 continue;
@@ -82,12 +82,13 @@ public class ReportService {
         return new DispatchResult(sent, skipped, mailEnabled && mailSenderProvider.getIfAvailable() != null);
     }
 
-    public byte[] reportPdf(String sampleId) {
-        return buildPdf(sampleService.loadSampleEntity(sampleId));
+    public byte[] reportPdf(String actorId, String sampleId) {
+        return buildPdf(sampleService.loadSampleEntity(actorId, sampleId));
     }
 
-    public SampleService.SampleDetail toggleReportBlocked(String sampleId, boolean blocked) {
-        return sampleService.getSample(sampleService.toggleReportBlocked(sampleId, blocked).id());
+    public SampleService.SampleDetail toggleReportBlocked(String actorId, String sampleId, boolean blocked) {
+        Sample sample = sampleService.loadSampleEntity(actorId, sampleId);
+        return sampleService.getSample(actorId, sampleService.toggleReportBlocked(sample.id(), blocked).id());
     }
 
     private void sendMail(Sample sample, byte[] pdf, String customerSignature) {
diff --git a/backend/src/main/java/de/svencarstensen/muh/service/SampleService.java b/backend/src/main/java/de/svencarstensen/muh/service/SampleService.java
index cec3a6e..a2f7266 100644
--- a/backend/src/main/java/de/svencarstensen/muh/service/SampleService.java
+++ b/backend/src/main/java/de/svencarstensen/muh/service/SampleService.java
@@ -1,6 +1,7 @@
 package de.svencarstensen.muh.service;
 
 import de.svencarstensen.muh.domain.AntibiogramEntry;
+import de.svencarstensen.muh.domain.AppUser;
 import de.svencarstensen.muh.domain.PathogenCatalogItem;
 import de.svencarstensen.muh.domain.PathogenKind;
 import de.svencarstensen.muh.domain.QuarterAntibiogram;
@@ -13,6 +14,8 @@ import de.svencarstensen.muh.domain.SamplingMode;
 import de.svencarstensen.muh.domain.SensitivityResult;
 import de.svencarstensen.muh.domain.TherapyRecommendation;
 import de.svencarstensen.muh.repository.SampleRepository;
+import de.svencarstensen.muh.repository.AppUserRepository;
+import de.svencarstensen.muh.security.AuthorizationService;
 import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Service;
 import org.springframework.web.server.ResponseStatusException;
@@ -25,33 +28,54 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
+import java.util.function.Function;
+import java.util.stream.Collectors;
 
 @Service
 public class SampleService {
 
     private final SampleRepository sampleRepository;
     private final CatalogService catalogService;
+    private final AppUserRepository appUserRepository;
+    private final AuthorizationService authorizationService;
 
-    public SampleService(SampleRepository sampleRepository, CatalogService catalogService) {
+    public SampleService(
+            SampleRepository sampleRepository,
+            CatalogService catalogService,
+            AppUserRepository appUserRepository,
+            AuthorizationService authorizationService
+    ) {
         this.sampleRepository = sampleRepository;
         this.catalogService = catalogService;
+        this.appUserRepository = appUserRepository;
+        this.authorizationService = authorizationService;
     }
 
-    public DashboardOverview dashboardOverview() {
-        List<SampleSummary> recent = sampleRepository.findTop12ByOrderByUpdatedAtDesc().stream()
+    public DashboardOverview dashboardOverview(String actorId) {
+        AppUser actor = requireActor(actorId);
+        List<Sample> accessibleSamples = accessibleSamples(actor);
+        List<SampleSummary> recent = accessibleSamples.stream()
+                .sorted(Comparator.comparing(Sample::updatedAt).reversed())
+                .limit(12)
                 .map(this::toSummary)
                 .toList();
-        long openCount = sampleRepository.findAll().stream().filter(sample -> sample.currentStep() != SampleWorkflowStep.COMPLETED).count();
+        long openCount = accessibleSamples.stream()
+                .filter(sample -> sample.currentStep() != SampleWorkflowStep.COMPLETED)
+                .count();
         LocalDate today = LocalDate.now();
-        long completedToday = sampleRepository.findByCompletedAtBetweenOrderByCompletedAtDesc(
-                today.atStartOfDay(),
-                today.plusDays(1).atStartOfDay()
-        ).size();
+        long completedToday = accessibleSamples.stream()
+                .filter(sample -> sample.completedAt() != null)
+                .filter(sample -> !sample.completedAt().isBefore(today.atStartOfDay()))
+                .filter(sample -> sample.completedAt().isBefore(today.plusDays(1).atStartOfDay()))
+                .count();
         return new DashboardOverview(nextSampleNumber(), openCount, completedToday, recent);
     }
 
-    public LookupResult lookup(long sampleNumber) {
+    public LookupResult lookup(String actorId, long sampleNumber) {
+        AppUser actor = requireActor(actorId);
         return sampleRepository.findBySampleNumber(sampleNumber)
+                .filter(sample -> canAccess(actor, sample))
                 .map(sample -> new LookupResult(
                         true,
                         "Probe gefunden",
@@ -62,17 +86,20 @@ public class SampleService {
                 .orElseGet(() -> new LookupResult(false, "Proben-Nummer unbekannt", null, null, null));
     }
 
-    public SampleDetail getSample(String id) {
-        return toDetail(loadSample(id));
+    public SampleDetail getSample(String actorId, String id) {
+        return toDetail(loadAccessibleSample(actorId, id));
     }
 
-    public SampleDetail getSampleByNumber(long sampleNumber) {
+    public SampleDetail getSampleByNumber(String actorId, long sampleNumber) {
+        AppUser actor = requireActor(actorId);
         return sampleRepository.findBySampleNumber(sampleNumber)
+                .filter(sample -> canAccess(actor, sample))
                 .map(this::toDetail)
                 .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Probe nicht gefunden"));
     }
 
-    public SampleDetail createSample(RegistrationRequest request) {
+    public SampleDetail createSample(String actorId, RegistrationRequest request) {
+        AppUser actor = requireActor(actorId);
         LocalDateTime now = LocalDateTime.now();
         CatalogService.FarmerOption farmer = catalogService.activeCatalogSummary().farmers().stream()
                 .filter(candidate -> candidate.businessKey().equals(request.farmerBusinessKey()))
@@ -99,6 +126,7 @@ public class SampleService {
                 now,
                 now,
                 null,
+                authorizationService.accountId(actor),
                 request.userCode(),
                 request.userDisplayName()
         );
@@ -106,8 +134,8 @@ public class SampleService {
         return toDetail(sampleRepository.save(sample));
     }
 
-    public SampleDetail saveRegistration(String id, RegistrationRequest request) {
-        Sample existing = loadSample(id);
+    public SampleDetail saveRegistration(String actorId, String id, RegistrationRequest request) {
+        Sample existing = loadAccessibleSample(actorId, id);
         if (!SampleWorkflowRules.canEditRegistration(existing)) {
             throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Stammdaten können nicht mehr geändert werden");
         }
@@ -137,6 +165,7 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 existing.completedAt(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
@@ -144,8 +173,8 @@ public class SampleService {
         return toDetail(saved);
     }
 
-    public SampleDetail saveAnamnesis(String id, AnamnesisRequest request) {
-        Sample existing = loadSample(id);
+    public SampleDetail saveAnamnesis(String actorId, String id, AnamnesisRequest request) {
+        Sample existing = loadAccessibleSample(actorId, id);
         if (!SampleWorkflowRules.canEditAnamnesis(existing)) {
             throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Anamnese kann an dieser Stelle nicht geändert werden");
         }
@@ -203,14 +232,15 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 existing.completedAt(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
         return toDetail(saved);
     }
 
-    public SampleDetail saveAntibiogram(String id, AntibiogramRequest request) {
-        Sample existing = loadSample(id);
+    public SampleDetail saveAntibiogram(String actorId, String id, AntibiogramRequest request) {
+        Sample existing = loadAccessibleSample(actorId, id);
         if (!SampleWorkflowRules.canEditAntibiogram(existing)) {
             throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Antibiogramm kann nicht mehr geändert werden");
         }
@@ -293,14 +323,15 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 existing.completedAt(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
         return toDetail(saved);
     }
 
-    public SampleDetail saveTherapy(String id, TherapyRequest request) {
-        Sample existing = loadSample(id);
+    public SampleDetail saveTherapy(String actorId, String id, TherapyRequest request) {
+        Sample existing = loadAccessibleSample(actorId, id);
         if (existing.currentStep() == SampleWorkflowStep.COMPLETED) {
             TherapyRecommendation previous = existing.therapyRecommendation();
             TherapyRecommendation updated = previous == null
@@ -341,6 +372,7 @@ public class SampleService {
                     existing.createdAt(),
                     LocalDateTime.now(),
                     existing.completedAt(),
+                    existing.ownerAccountId(),
                     existing.createdByUserCode(),
                     existing.createdByDisplayName()
             )));
@@ -388,6 +420,7 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 LocalDateTime.now(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
@@ -416,6 +449,7 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 existing.completedAt(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
@@ -443,25 +477,41 @@ public class SampleService {
                 existing.createdAt(),
                 LocalDateTime.now(),
                 existing.completedAt(),
+                existing.ownerAccountId(),
                 existing.createdByUserCode(),
                 existing.createdByDisplayName()
         ));
     }
 
-    public List<Sample> completedSamples() {
-        return sampleRepository.findByCompletedAtNotNullOrderByCompletedAtDesc();
+    public List<Sample> completedSamples(String actorId) {
+        return accessibleSamples(requireActor(actorId)).stream()
+                .filter(sample -> sample.completedAt() != null)
+                .sorted(Comparator.comparing(Sample::completedAt).reversed())
+                .toList();
     }
 
-    public List<Sample> samplesByFarmerBusinessKey(String businessKey) {
-        return sampleRepository.findByFarmerBusinessKeyOrderByCreatedAtDesc(businessKey);
+    public List<Sample> samplesByFarmerBusinessKey(String actorId, String businessKey) {
+        return accessibleSamples(requireActor(actorId)).stream()
+                .filter(sample -> Objects.equals(sample.farmerBusinessKey(), businessKey))
+                .sorted(Comparator.comparing(Sample::createdAt).reversed())
+                .toList();
     }
 
-    public List<Sample> samplesByCreatedDate(LocalDate date) {
-        return sampleRepository.findByCreatedAtBetweenOrderByCreatedAtDesc(date.atStartOfDay(), date.plusDays(1).atStartOfDay());
+    public List<Sample> samplesByCreatedDate(String actorId, LocalDate date) {
+        return accessibleSamples(requireActor(actorId)).stream()
+                .filter(sample -> !sample.createdAt().isBefore(date.atStartOfDay()))
+                .filter(sample -> sample.createdAt().isBefore(date.plusDays(1).atStartOfDay()))
+                .sorted(Comparator.comparing(Sample::createdAt).reversed())
+                .toList();
     }
 
-    public List<Sample> samplesByDate(LocalDate date) {
-        return sampleRepository.findByCompletedAtBetweenOrderByCompletedAtDesc(date.atStartOfDay(), date.plusDays(1).atStartOfDay());
+    public List<Sample> samplesByDate(String actorId, LocalDate date) {
+        return accessibleSamples(requireActor(actorId)).stream()
+                .filter(sample -> sample.completedAt() != null)
+                .filter(sample -> !sample.completedAt().isBefore(date.atStartOfDay()))
+                .filter(sample -> sample.completedAt().isBefore(date.plusDays(1).atStartOfDay()))
+                .sorted(Comparator.comparing(Sample::completedAt).reversed())
+                .toList();
     }
 
     public long nextSampleNumber() {
@@ -470,15 +520,119 @@ public class SampleService {
                 .orElse(100001L);
     }
 
-    public Sample loadSampleEntity(String id) {
-        return loadSample(id);
+    public Sample loadSampleEntity(String actorId, String id) {
+        return loadAccessibleSample(actorId, id);
+    }
+
+    private AppUser requireActor(String actorId) {
+        ensureSampleOwnershipMigration();
+        return authorizationService.requireActiveUser(actorId, "Nicht berechtigt");
+    }
+
+    private List<Sample> accessibleSamples(AppUser actor) {
+        List<Sample> samples = sampleRepository.findAll();
+        if (authorizationService.isAdmin(actor)) {
+            return samples;
+        }
+        String accountId = authorizationService.accountId(actor);
+        return samples.stream()
+                .filter(sample -> Objects.equals(sample.ownerAccountId(), accountId))
+                .toList();
+    }
+
+    private boolean canAccess(AppUser actor, Sample sample) {
+        return authorizationService.isAdmin(actor)
+                || Objects.equals(sample.ownerAccountId(), authorizationService.accountId(actor));
+    }
+
+    private Sample loadAccessibleSample(String actorId, String id) {
+        AppUser actor = requireActor(actorId);
+        Sample sample = loadSample(id);
+        if (!canAccess(actor, sample)) {
+            throw new ResponseStatusException(HttpStatus.NOT_FOUND, "Probe nicht gefunden");
+        }
+        return sample;
     }
 
     private Sample loadSample(String id) {
+        ensureSampleOwnershipMigration();
         return sampleRepository.findById(Objects.requireNonNull(id))
                 .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Probe nicht gefunden"));
     }
 
+    private void ensureSampleOwnershipMigration() {
+        List<Sample> samples = sampleRepository.findAll().stream()
+                .filter(sample -> sample.ownerAccountId() == null || sample.ownerAccountId().isBlank())
+                .toList();
+        if (samples.isEmpty()) {
+            return;
+        }
+
+        List<AppUser> users = appUserRepository.findAll().stream()
+                .filter(AppUser::active)
+                .toList();
+        Map<String, List<AppUser>> usersByDisplayName = users.stream()
+                .filter(user -> user.displayName() != null && !user.displayName().isBlank())
+                .collect(Collectors.groupingBy(user -> user.displayName().trim().toLowerCase()));
+        List<String> primaryCustomerAccounts = users.stream()
+                .filter(user -> user.role() != de.svencarstensen.muh.domain.UserRole.ADMIN)
+                .filter(user -> Boolean.TRUE.equals(user.primaryUser()))
+                .map(authorizationService::accountId)
+                .distinct()
+                .toList();
+        String fallbackAccountId = primaryCustomerAccounts.size() == 1 ? primaryCustomerAccounts.get(0) : null;
+
+        for (Sample sample : samples) {
+            String resolvedAccountId = resolveSampleOwnerAccountId(sample, usersByDisplayName, fallbackAccountId);
+            if (resolvedAccountId == null) {
+                continue;
+            }
+            sampleRepository.save(new Sample(
+                    sample.id(),
+                    sample.sampleNumber(),
+                    sample.farmerBusinessKey(),
+                    sample.farmerName(),
+                    sample.farmerEmail(),
+                    sample.cowNumber(),
+                    sample.cowName(),
+                    sample.sampleKind(),
+                    sample.samplingMode(),
+                    sample.currentStep(),
+                    sample.quarters(),
+                    sample.antibiograms(),
+                    sample.therapyRecommendation(),
+                    sample.reportSent(),
+                    sample.reportBlocked(),
+                    sample.reportSentAt(),
+                    sample.createdAt(),
+                    sample.updatedAt(),
+                    sample.completedAt(),
+                    resolvedAccountId,
+                    sample.createdByUserCode(),
+                    sample.createdByDisplayName()
+            ));
+        }
+    }
+
+    private String resolveSampleOwnerAccountId(
+            Sample sample,
+            Map<String, List<AppUser>> usersByDisplayName,
+            String fallbackAccountId
+    ) {
+        if (sample.createdByDisplayName() != null && !sample.createdByDisplayName().isBlank()) {
+            List<String> matchingAccounts = usersByDisplayName
+                    .getOrDefault(sample.createdByDisplayName().trim().toLowerCase(), List.of())
+                    .stream()
+                    .map(authorizationService::accountId)
+                    .distinct()
+                    .toList();
+            if (matchingAccounts.size() == 1) {
+                return matchingAccounts.get(0);
+            }
+        }
+        return fallbackAccountId;
+    }
+
     private SampleSummary toSummary(Sample sample) {
         return new SampleSummary(
                 sample.id(),
diff --git a/backend/src/main/java/de/svencarstensen/muh/web/CatalogController.java b/backend/src/main/java/de/svencarstensen/muh/web/CatalogController.java
index 0a1cce4..1d1555d 100644
--- a/backend/src/main/java/de/svencarstensen/muh/web/CatalogController.java
+++ b/backend/src/main/java/de/svencarstensen/muh/web/CatalogController.java
@@ -32,27 +32,27 @@ public class CatalogController {
 
     @GetMapping("/admin")
     public CatalogService.AdministrationOverview administrationOverview() {
-        return catalogService.administrationOverview();
+        return catalogService.administrationOverview(securitySupport.currentUser().id());
     }
 
     @PostMapping("/admin/farmers")
     public List<CatalogService.FarmerRow> saveFarmers(@RequestBody List<CatalogService.FarmerMutation> mutations) {
-        return catalogService.saveFarmers(mutations);
+        return catalogService.saveFarmers(securitySupport.currentUser().id(), mutations);
     }
 
     @PostMapping("/admin/medications")
     public List<CatalogService.MedicationRow> saveMedications(@RequestBody List<CatalogService.MedicationMutation> mutations) {
-        return catalogService.saveMedications(mutations);
+        return catalogService.saveMedications(securitySupport.currentUser().id(), mutations);
     }
 
     @PostMapping("/admin/pathogens")
     public List<CatalogService.PathogenRow> savePathogens(@RequestBody List<CatalogService.PathogenMutation> mutations) {
-        return catalogService.savePathogens(mutations);
+        return catalogService.savePathogens(securitySupport.currentUser().id(), mutations);
     }
 
     @PostMapping("/admin/antibiotics")
     public List<CatalogService.AntibioticRow> saveAntibiotics(@RequestBody List<CatalogService.AntibioticMutation> mutations) {
-        return catalogService.saveAntibiotics(mutations);
+        return catalogService.saveAntibiotics(securitySupport.currentUser().id(), mutations);
     }
 
     @GetMapping("/portal/users")
diff --git a/backend/src/main/java/de/svencarstensen/muh/web/PortalController.java b/backend/src/main/java/de/svencarstensen/muh/web/PortalController.java
index 01f5ed4..da29ced 100644
--- a/backend/src/main/java/de/svencarstensen/muh/web/PortalController.java
+++ b/backend/src/main/java/de/svencarstensen/muh/web/PortalController.java
@@ -58,12 +58,12 @@ public class PortalController {
 
     @GetMapping("/reports")
     public List<ReportService.ReportCandidate> reports() {
-        return reportService.reportCandidates();
+        return reportService.reportCandidates(securitySupport.currentUser().id());
     }
 
     @GetMapping("/search/by-date")
     public List<PortalService.PortalSampleRow> searchByDate(@RequestParam LocalDate date) {
-        return portalService.searchSamplesByCreatedDate(date);
+        return portalService.searchSamplesByCreatedDate(securitySupport.currentUser().id(), date);
     }
 
     @PostMapping("/reports/send")
@@ -73,12 +73,12 @@ public class PortalController {
 
     @PatchMapping("/reports/{sampleId}/block")
     public SampleService.SampleDetail block(@PathVariable String sampleId, @RequestBody BlockRequest request) {
-        return reportService.toggleReportBlocked(sampleId, request.blocked());
+        return reportService.toggleReportBlocked(securitySupport.currentUser().id(), sampleId, request.blocked());
     }
 
     @GetMapping("/reports/{sampleId}/pdf")
     public ResponseEntity<byte[]> pdf(@PathVariable String sampleId) {
-        byte[] pdf = reportService.reportPdf(sampleId);
+        byte[] pdf = reportService.reportPdf(securitySupport.currentUser().id(), sampleId);
         return ResponseEntity.ok()
                 .contentType(Objects.requireNonNull(MediaType.APPLICATION_PDF))
                 .header(HttpHeaders.CONTENT_DISPOSITION, ContentDisposition.inline()
diff --git a/backend/src/main/java/de/svencarstensen/muh/web/SampleController.java b/backend/src/main/java/de/svencarstensen/muh/web/SampleController.java
index b33ca87..7e211d6 100644
--- a/backend/src/main/java/de/svencarstensen/muh/web/SampleController.java
+++ b/backend/src/main/java/de/svencarstensen/muh/web/SampleController.java
@@ -25,28 +25,28 @@ public class SampleController {
 
     @GetMapping("/dashboard")
     public SampleService.DashboardOverview dashboardOverview() {
-        return sampleService.dashboardOverview();
+        return sampleService.dashboardOverview(securitySupport.currentUser().id());
     }
 
     @GetMapping("/dashboard/lookup/{sampleNumber}")
     public SampleService.LookupResult lookup(@PathVariable long sampleNumber) {
-        return sampleService.lookup(sampleNumber);
+        return sampleService.lookup(securitySupport.currentUser().id(), sampleNumber);
     }
 
     @GetMapping("/samples/{id}")
     public SampleService.SampleDetail sample(@PathVariable String id) {
-        return sampleService.getSample(id);
+        return sampleService.getSample(securitySupport.currentUser().id(), id);
     }
 
     @GetMapping("/samples/by-number/{sampleNumber}")
     public SampleService.SampleDetail sampleByNumber(@PathVariable long sampleNumber) {
-        return sampleService.getSampleByNumber(sampleNumber);
+        return sampleService.getSampleByNumber(securitySupport.currentUser().id(), sampleNumber);
     }
 
     @PostMapping("/samples")
     public SampleService.SampleDetail create(@RequestBody SampleService.RegistrationRequest request) {
         AuthenticatedUser user = securitySupport.currentUser();
-        return sampleService.createSample(new SampleService.RegistrationRequest(
+        return sampleService.createSample(user.id(), new SampleService.RegistrationRequest(
                 request.farmerBusinessKey(),
                 request.cowNumber(),
                 request.cowName(),
@@ -60,22 +60,22 @@ public class SampleController {
 
     @PutMapping("/samples/{id}/registration")
     public SampleService.SampleDetail saveRegistration(@PathVariable String id, @RequestBody SampleService.RegistrationRequest request) {
-        return sampleService.saveRegistration(id, request);
+        return sampleService.saveRegistration(securitySupport.currentUser().id(), id, request);
     }
 
     @PutMapping("/samples/{id}/anamnesis")
     public SampleService.SampleDetail saveAnamnesis(@PathVariable String id, @RequestBody SampleService.AnamnesisRequest request) {
-        return sampleService.saveAnamnesis(id, request);
+        return sampleService.saveAnamnesis(securitySupport.currentUser().id(), id, request);
     }
 
     @PutMapping("/samples/{id}/antibiogram")
     public SampleService.SampleDetail saveAntibiogram(@PathVariable String id, @RequestBody SampleService.AntibiogramRequest request) {
-        return sampleService.saveAntibiogram(id, request);
+        return sampleService.saveAntibiogram(securitySupport.currentUser().id(), id, request);
     }
 
     @PutMapping("/samples/{id}/therapy")
     public SampleService.SampleDetail saveTherapy(@PathVariable String id, @RequestBody SampleService.TherapyRequest request) {
-        return sampleService.saveTherapy(id, request);
+        return sampleService.saveTherapy(securitySupport.currentUser().id(), id, request);
     }
 
     private String deriveUserLabel(String displayName) {
diff --git a/backend/src/main/resources/application.yml b/backend/src/main/resources/application.yml
index 9a953c3..56b7d16 100644
--- a/backend/src/main/resources/application.yml
+++ b/backend/src/main/resources/application.yml
@@ -34,7 +34,7 @@ muh:
   cors:
     allowed-origins: ${MUH_ALLOWED_ORIGINS:http://localhost:5173,http://localhost:3000}
   security:
-    token-secret: ${MUH_TOKEN_SECRET:change-me-in-production}
+    token-secret: ${MUH_TOKEN_SECRET:}
     token-validity-hours: ${MUH_TOKEN_VALIDITY_HOURS:12}
   mongodb:
     url: ${MUH_MONGODB_URL:mongodb://192.168.180.25:27017/muh}
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index e77a182..a4fac72 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -16,6 +16,7 @@ import UserManagementPage from "./pages/UserManagementPage";
 
 function ProtectedRoutes() {
   const { user, ready } = useSession();
+  const isAdmin = user?.role === "ADMIN";
 
   if (!ready) {
     return <div className="empty-state">Sitzung wird geladen ...</div>;
@@ -34,7 +35,7 @@ function ProtectedRoutes() {
         <Route path="/samples/:sampleId/anamnesis" element={<AnamnesisPage />} />
         <Route path="/samples/:sampleId/antibiogram" element={<AntibiogramPage />} />
         <Route path="/samples/:sampleId/therapy" element={<TherapyPage />} />
-        <Route path="/admin" element={<Navigate to="/admin/landwirte" replace />} />
+        <Route path="/admin" element={<Navigate to={isAdmin ? "/admin/landwirte" : "/admin/benutzer"} replace />} />
         <Route path="/admin/benutzer" element={<UserManagementPage />} />
         <Route path="/admin/landwirte" element={<AdministrationPage />} />
         <Route path="/admin/medikamente" element={<AdministrationPage />} />
diff --git a/frontend/src/layout/AppShell.tsx b/frontend/src/layout/AppShell.tsx
index b82e1db..b2fcd85 100644
--- a/frontend/src/layout/AppShell.tsx
+++ b/frontend/src/layout/AppShell.tsx
@@ -21,19 +21,19 @@ function resolvePageTitle(pathname: string) {
     return "Probe bearbeiten";
   }
   if (pathname.startsWith("/admin/landwirte")) {
-    return "Verwaltung | Landwirte";
+    return "Die Verwaltung der Landwirte";
   }
   if (pathname.startsWith("/admin/benutzer")) {
     return "Verwaltung | Benutzer";
   }
   if (pathname.startsWith("/admin/medikamente")) {
-    return "Verwaltung | Medikamente";
+    return "Die Verwaltung der Medikamente";
   }
   if (pathname.startsWith("/admin/erreger")) {
-    return "Verwaltung | Erreger";
+    return "Die Verwaltung der Erreger";
   }
   if (pathname.startsWith("/admin/antibiogramm")) {
-    return "Verwaltung | Antibiogramm";
+    return "Die Verwaltung der Antibiogramme";
   }
   if (pathname.startsWith("/search/landwirt")) {
     return "Suche | Landwirt";
@@ -79,9 +79,6 @@ export default function AppShell() {
               <div className="nav-group">
                 <div className="nav-group__label">Verwaltung</div>
                 <div className="nav-subnav">
-                  <NavLink to="/admin/benutzer" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
-                    Benutzer
-                  </NavLink>
                   <NavLink to="/admin/landwirte" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
                     Landwirte
                   </NavLink>
@@ -94,6 +91,9 @@ export default function AppShell() {
                   <NavLink to="/admin/antibiogramm" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
                     Antibiogramm
                   </NavLink>
+                  <NavLink to="/admin/benutzer" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
+                    Benutzer
+                  </NavLink>
                 </div>
               </div>
 
diff --git a/frontend/src/pages/AdministrationPage.tsx b/frontend/src/pages/AdministrationPage.tsx
index bac786a..bc72fd2 100644
--- a/frontend/src/pages/AdministrationPage.tsx
+++ b/frontend/src/pages/AdministrationPage.tsx
@@ -26,6 +26,13 @@ const DATASET_LABELS: Record<DatasetKey, string> = {
   antibiotics: "Antibiogramm",
 };
 
+const DATASET_TITLES: Record<DatasetKey, string> = {
+  farmers: "Die Verwaltung der Landwirte",
+  medications: "Die Verwaltung der Medikamente",
+  pathogens: "Die Verwaltung der Erreger",
+  antibiotics: "Die Verwaltung der Antibiogramme",
+};
+
 function normalizeOverview(overview: AdministrationOverview): DatasetsState {
   return {
     farmers: overview.farmers.map((entry) => ({
@@ -216,7 +223,7 @@ export default function AdministrationPage() {
       <section className="section-card section-card--hero">
         <div>
           <p className="eyebrow">Verwaltung</p>
-          <h3>Stammdaten direkt pflegen</h3>
+          <h3>{DATASET_TITLES[selectedDataset]}</h3>
           <p className="muted-text">
             Bestehende Datensaetze lassen sich inline aendern. Bei Umbenennungen bleibt der alte
             Satz inaktiv sichtbar.