diff --git a/app/Jobs/GenerateDriftFindingsJob.php b/app/Jobs/GenerateDriftFindingsJob.php
index 8c0de42..645dae4 100644
--- a/app/Jobs/GenerateDriftFindingsJob.php
+++ b/app/Jobs/GenerateDriftFindingsJob.php
@@ -2,11 +2,15 @@
 
 namespace App\Jobs;
 
+use App\Models\InventorySyncRun;
+use App\Models\Tenant;
+use App\Services\Drift\DriftFindingGenerator;
 use Illuminate\Bus\Queueable;
 use Illuminate\Contracts\Queue\ShouldQueue;
 use Illuminate\Foundation\Bus\Dispatchable;
 use Illuminate\Queue\InteractsWithQueue;
 use Illuminate\Queue\SerializesModels;
+use RuntimeException;
 
 class GenerateDriftFindingsJob implements ShouldQueue
 {
@@ -23,8 +27,28 @@ public function __construct(
     /**
      * Execute the job.
      */
-    public function handle(): void
+    public function handle(DriftFindingGenerator $generator): void
     {
-        // Implemented in later tasks (T020/T021).
+        $tenant = Tenant::query()->find($this->tenantId);
+        if (! $tenant instanceof Tenant) {
+            throw new RuntimeException('Tenant not found.');
+        }
+
+        $baseline = InventorySyncRun::query()->find($this->baselineRunId);
+        if (! $baseline instanceof InventorySyncRun) {
+            throw new RuntimeException('Baseline run not found.');
+        }
+
+        $current = InventorySyncRun::query()->find($this->currentRunId);
+        if (! $current instanceof InventorySyncRun) {
+            throw new RuntimeException('Current run not found.');
+        }
+
+        $generator->generate(
+            tenant: $tenant,
+            baseline: $baseline,
+            current: $current,
+            scopeKey: $this->scopeKey,
+        );
     }
 }
diff --git a/app/Services/Drift/DriftFindingGenerator.php b/app/Services/Drift/DriftFindingGenerator.php
new file mode 100644
index 0000000..1fc88fa
--- /dev/null
+++ b/app/Services/Drift/DriftFindingGenerator.php
@@ -0,0 +1,127 @@
+<?php
+
+namespace App\Services\Drift;
+
+use App\Models\Finding;
+use App\Models\InventorySyncRun;
+use App\Models\Policy;
+use App\Models\PolicyVersion;
+use App\Models\Tenant;
+use Illuminate\Support\Arr;
+use RuntimeException;
+
+class DriftFindingGenerator
+{
+    public function __construct(
+        private readonly DriftHasher $hasher,
+        private readonly DriftEvidence $evidence,
+    ) {}
+
+    public function generate(Tenant $tenant, InventorySyncRun $baseline, InventorySyncRun $current, string $scopeKey): int
+    {
+        if (! $baseline->finished_at || ! $current->finished_at) {
+            throw new RuntimeException('Baseline/current run must be finished.');
+        }
+
+        /** @var array<string, mixed> $selection */
+        $selection = is_array($current->selection_payload) ? $current->selection_payload : [];
+
+        $policyTypes = Arr::get($selection, 'policy_types');
+        if (! is_array($policyTypes)) {
+            $policyTypes = [];
+        }
+
+        $policyTypes = array_values(array_filter(array_map('strval', $policyTypes)));
+
+        $created = 0;
+
+        Policy::query()
+            ->where('tenant_id', $tenant->getKey())
+            ->whereIn('policy_type', $policyTypes)
+            ->orderBy('id')
+            ->chunk(200, function ($policies) use ($tenant, $baseline, $current, $scopeKey, &$created): void {
+                foreach ($policies as $policy) {
+                    if (! $policy instanceof Policy) {
+                        continue;
+                    }
+
+                    $baselineVersion = $this->versionForRun($policy, $baseline);
+                    $currentVersion = $this->versionForRun($policy, $current);
+
+                    if (! $baselineVersion instanceof PolicyVersion || ! $currentVersion instanceof PolicyVersion) {
+                        continue;
+                    }
+
+                    $baselineAssignmentsHash = $baselineVersion->assignments_hash ?? null;
+                    $currentAssignmentsHash = $currentVersion->assignments_hash ?? null;
+
+                    if ($baselineAssignmentsHash === $currentAssignmentsHash) {
+                        continue;
+                    }
+
+                    $fingerprint = $this->hasher->fingerprint(
+                        tenantId: (int) $tenant->getKey(),
+                        scopeKey: $scopeKey,
+                        subjectType: 'assignment',
+                        subjectExternalId: (string) $policy->external_id,
+                        changeType: 'modified',
+                        baselineHash: (string) ($baselineAssignmentsHash ?? ''),
+                        currentHash: (string) ($currentAssignmentsHash ?? ''),
+                    );
+
+                    $rawEvidence = [
+                        'change_type' => 'modified',
+                        'summary' => 'Policy assignments changed',
+                        'baseline' => [
+                            'policy_id' => $policy->external_id,
+                            'policy_version_id' => $baselineVersion->getKey(),
+                            'assignments_hash' => $baselineAssignmentsHash,
+                        ],
+                        'current' => [
+                            'policy_id' => $policy->external_id,
+                            'policy_version_id' => $currentVersion->getKey(),
+                            'assignments_hash' => $currentAssignmentsHash,
+                        ],
+                    ];
+
+                    Finding::query()->updateOrCreate(
+                        [
+                            'tenant_id' => $tenant->getKey(),
+                            'fingerprint' => $fingerprint,
+                        ],
+                        [
+                            'finding_type' => Finding::FINDING_TYPE_DRIFT,
+                            'scope_key' => $scopeKey,
+                            'baseline_run_id' => $baseline->getKey(),
+                            'current_run_id' => $current->getKey(),
+                            'subject_type' => 'assignment',
+                            'subject_external_id' => (string) $policy->external_id,
+                            'severity' => Finding::SEVERITY_MEDIUM,
+                            'status' => Finding::STATUS_NEW,
+                            'acknowledged_at' => null,
+                            'acknowledged_by_user_id' => null,
+                            'evidence_jsonb' => $this->evidence->sanitize($rawEvidence),
+                        ],
+                    );
+
+                    $created++;
+                }
+            });
+
+        return $created;
+    }
+
+    private function versionForRun(Policy $policy, InventorySyncRun $run): ?PolicyVersion
+    {
+        if (! $run->finished_at) {
+            return null;
+        }
+
+        return PolicyVersion::query()
+            ->where('tenant_id', $policy->tenant_id)
+            ->where('policy_id', $policy->getKey())
+            ->where('captured_at', '<=', $run->finished_at)
+            ->latest('captured_at')
+            ->first();
+    }
+}
diff --git a/tests/Feature/Drift/DriftAssignmentDriftDetectionTest.php b/tests/Feature/Drift/DriftAssignmentDriftDetectionTest.php
new file mode 100644
index 0000000..5a186ea
--- /dev/null
+++ b/tests/Feature/Drift/DriftAssignmentDriftDetectionTest.php
@@ -0,0 +1,76 @@
+<?php
+
+use App\Models\Finding;
+use App\Models\InventorySyncRun;
+use App\Models\Policy;
+use App\Models\PolicyVersion;
+use App\Services\Drift\DriftFindingGenerator;
+
+test('it creates a drift finding when policy assignment targets change', function () {
+    [$user, $tenant] = createUserWithTenant(role: 'manager');
+
+    $scopeKey = hash('sha256', 'scope-assignments');
+
+    $baseline = InventorySyncRun::factory()->for($tenant)->create([
+        'selection_hash' => $scopeKey,
+        'selection_payload' => ['policy_types' => ['settingsCatalogPolicy']],
+        'status' => InventorySyncRun::STATUS_SUCCESS,
+        'finished_at' => now()->subDays(2),
+    ]);
+
+    $current = InventorySyncRun::factory()->for($tenant)->create([
+        'selection_hash' => $scopeKey,
+        'selection_payload' => ['policy_types' => ['settingsCatalogPolicy']],
+        'status' => InventorySyncRun::STATUS_SUCCESS,
+        'finished_at' => now()->subDay(),
+    ]);
+
+    $policy = Policy::factory()->for($tenant)->create([
+        'policy_type' => 'settingsCatalogPolicy',
+    ]);
+
+    $baselineAssignments = [
+        [
+            'target' => [
+                '@odata.type' => '#microsoft.graph.groupAssignmentTarget',
+                'groupId' => 'group-a',
+            ],
+        ],
+    ];
+
+    $currentAssignments = [
+        [
+            'target' => [
+                '@odata.type' => '#microsoft.graph.groupAssignmentTarget',
+                'groupId' => 'group-b',
+            ],
+        ],
+    ];
+
+    PolicyVersion::factory()->for($tenant)->for($policy)->create([
+        'version_number' => 1,
+        'policy_type' => $policy->policy_type,
+        'captured_at' => $baseline->finished_at->copy()->subMinute(),
+        'assignments' => $baselineAssignments,
+        'assignments_hash' => hash('sha256', json_encode($baselineAssignments)),
+    ]);
+
+    PolicyVersion::factory()->for($tenant)->for($policy)->create([
+        'version_number' => 2,
+        'policy_type' => $policy->policy_type,
+        'captured_at' => $current->finished_at->copy()->subMinute(),
+        'assignments' => $currentAssignments,
+        'assignments_hash' => hash('sha256', json_encode($currentAssignments)),
+    ]);
+
+    $generator = app(DriftFindingGenerator::class);
+    $created = $generator->generate($tenant, $baseline, $current, $scopeKey);
+
+    expect($created)->toBe(1);
+
+    $finding = Finding::query()->where('tenant_id', $tenant->getKey())->first();
+    expect($finding)->not->toBeNull();
+    expect($finding->subject_type)->toBe('assignment');
+    expect($finding->subject_external_id)->toBe($policy->external_id);
+    expect($finding->evidence_jsonb)->toHaveKey('change_type', 'modified');
+});
diff --git a/tests/Feature/Drift/DriftTenantIsolationTest.php b/tests/Feature/Drift/DriftTenantIsolationTest.php
new file mode 100644
index 0000000..a6661a2
--- /dev/null
+++ b/tests/Feature/Drift/DriftTenantIsolationTest.php
@@ -0,0 +1,80 @@
+<?php
+
+use App\Models\Finding;
+use App\Models\InventorySyncRun;
+use App\Models\Policy;
+use App\Models\PolicyVersion;
+use App\Services\Drift\DriftFindingGenerator;
+
+test('drift generation is tenant isolated', function () {
+    [$userA, $tenantA] = createUserWithTenant(role: 'manager');
+    [$userB, $tenantB] = createUserWithTenant(role: 'manager');
+
+    $scopeKey = hash('sha256', 'scope-tenant');
+
+    $baselineA = InventorySyncRun::factory()->for($tenantA)->create([
+        'selection_hash' => $scopeKey,
+        'selection_payload' => ['policy_types' => ['settingsCatalogPolicy']],
+        'status' => InventorySyncRun::STATUS_SUCCESS,
+        'finished_at' => now()->subDays(2),
+    ]);
+
+    $currentA = InventorySyncRun::factory()->for($tenantA)->create([
+        'selection_hash' => $scopeKey,
+        'selection_payload' => ['policy_types' => ['settingsCatalogPolicy']],
+        'status' => InventorySyncRun::STATUS_SUCCESS,
+        'finished_at' => now()->subDay(),
+    ]);
+
+    $policyA = Policy::factory()->for($tenantA)->create([
+        'policy_type' => 'settingsCatalogPolicy',
+    ]);
+
+    $baselineAssignments = [['target' => ['groupId' => 'group-a'], '@odata.type' => '#microsoft.graph.groupAssignmentTarget']];
+    $currentAssignments = [['target' => ['groupId' => 'group-b'], '@odata.type' => '#microsoft.graph.groupAssignmentTarget']];
+
+    PolicyVersion::factory()->for($tenantA)->for($policyA)->create([
+        'version_number' => 1,
+        'policy_type' => $policyA->policy_type,
+        'captured_at' => $baselineA->finished_at->copy()->subMinute(),
+        'assignments' => $baselineAssignments,
+        'assignments_hash' => hash('sha256', json_encode($baselineAssignments)),
+    ]);
+
+    PolicyVersion::factory()->for($tenantA)->for($policyA)->create([
+        'version_number' => 2,
+        'policy_type' => $policyA->policy_type,
+        'captured_at' => $currentA->finished_at->copy()->subMinute(),
+        'assignments' => $currentAssignments,
+        'assignments_hash' => hash('sha256', json_encode($currentAssignments)),
+    ]);
+
+    $policyB = Policy::factory()->for($tenantB)->create([
+        'policy_type' => 'settingsCatalogPolicy',
+    ]);
+
+    $baselineAssignmentsB = [['target' => ['groupId' => 'group-x'], '@odata.type' => '#microsoft.graph.groupAssignmentTarget']];
+    $currentAssignmentsB = [['target' => ['groupId' => 'group-y'], '@odata.type' => '#microsoft.graph.groupAssignmentTarget']];
+
+    PolicyVersion::factory()->for($tenantB)->for($policyB)->create([
+        'version_number' => 1,
+        'policy_type' => $policyB->policy_type,
+        'captured_at' => now()->subDays(2)->subMinute(),
+        'assignments' => $baselineAssignmentsB,
+        'assignments_hash' => hash('sha256', json_encode($baselineAssignmentsB)),
+    ]);
+
+    PolicyVersion::factory()->for($tenantB)->for($policyB)->create([
+        'version_number' => 2,
+        'policy_type' => $policyB->policy_type,
+        'captured_at' => now()->subDay()->subMinute(),
+        'assignments' => $currentAssignmentsB,
+        'assignments_hash' => hash('sha256', json_encode($currentAssignmentsB)),
+    ]);
+
+    $generator = app(DriftFindingGenerator::class);
+    $generator->generate($tenantA, $baselineA, $currentA, $scopeKey);
+
+    expect(Finding::query()->where('tenant_id', $tenantA->getKey())->count())->toBe(1);
+    expect(Finding::query()->where('tenant_id', $tenantB->getKey())->count())->toBe(0);
+});