diff --git a/services/domain/Big.Tests/ApproveRegistrationTests.cs b/services/domain/Big.Tests/ApproveRegistrationTests.cs
new file mode 100644
index 0000000..2112f96
--- /dev/null
+++ b/services/domain/Big.Tests/ApproveRegistrationTests.cs
@@ -0,0 +1,74 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class ApproveRegistrationTests
+{
+    private static Registration WithZaak(string bsn = "123456782")
+    {
+        var registration = Registration.Submit(bsn);
+        registration.AttachZaak(FakeAclClient.DefaultZaakUrl);
+        return registration;
+    }
+
+    [Fact]
+    public async Task Approving_sets_the_zaak_status_via_the_acl_and_marks_the_registration_ingeschreven()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = WithZaak();
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+
+        var saved = await store.GetAsync(registration.Id);
+        Assert.Equal(RegistrationStatus.Ingeschreven, saved!.Status);
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, acl.ApprovedZaakUrl);
+        Assert.Equal(1, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Approving_an_unknown_registration_throws_and_does_not_call_the_acl()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var handler = new ApproveRegistration(store, acl);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new ApproveRegistrationCommand(RegistrationId.New())));
+        Assert.Equal(0, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Approving_before_the_zaak_is_opened_throws_without_calling_the_acl()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = Registration.Submit("123456782"); // no zaak yet
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new ApproveRegistrationCommand(registration.Id)));
+        Assert.Equal(0, acl.ApproveCallCount);
+    }
+
+    [Fact]
+    public async Task Re_approving_an_already_ingeschreven_registration_is_idempotent()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var registration = WithZaak();
+        store.Seed(registration);
+        var handler = new ApproveRegistration(store, acl);
+
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+        await handler.HandleAsync(new ApproveRegistrationCommand(registration.Id));
+
+        // The second approval is a no-op: the ACL is not asked to set the status again.
+        Assert.Equal(1, acl.ApproveCallCount);
+        Assert.Equal(RegistrationStatus.Ingeschreven, (await store.GetAsync(registration.Id))!.Status);
+    }
+}
diff --git a/services/domain/Big.Tests/Fakes.cs b/services/domain/Big.Tests/Fakes.cs
index 38d1bae..2377b7b 100644
--- a/services/domain/Big.Tests/Fakes.cs
+++ b/services/domain/Big.Tests/Fakes.cs
@@ -52,10 +52,20 @@ internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
     public string? OpenedForBsn { get; private set; }
     public int CallCount { get; private set; }
 
+    public Uri? ApprovedZaakUrl { get; private set; }
+    public int ApproveCallCount { get; private set; }
+
     public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
     {
         CallCount++;
         OpenedForBsn = bsn;
         return Task.FromResult(_zaakUrl);
     }
+
+    public Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ApproveCallCount++;
+        ApprovedZaakUrl = zaakUrl;
+        return Task.CompletedTask;
+    }
 }