diff --git a/register-referentie.slnx b/register-referentie.slnx
index 40d5e27..072c997 100644
--- a/register-referentie.slnx
+++ b/register-referentie.slnx
@@ -9,6 +9,7 @@
   </Folder>
   <Folder Name="/services/domain/">
     <Project Path="services/domain/Big.Domain/Big.Domain.csproj" />
+    <Project Path="services/domain/Big.Application/Big.Application.csproj" />
     <Project Path="services/domain/Big.Tests/Big.Tests.csproj" />
   </Folder>
   <Folder Name="/services/bff/">
diff --git a/services/domain/Big.Application/Big.Application.csproj b/services/domain/Big.Application/Big.Application.csproj
new file mode 100644
index 0000000..b877d01
--- /dev/null
+++ b/services/domain/Big.Application/Big.Application.csproj
@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!-- The application layer (CLAUDE.md §9): use cases over ports. Depends on Domain only;
+       Infrastructure implements the ports. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Domain\Big.Domain.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/services/domain/Big.Application/OpenZaakWorker.cs b/services/domain/Big.Application/OpenZaakWorker.cs
new file mode 100644
index 0000000..6f01c36
--- /dev/null
+++ b/services/domain/Big.Application/OpenZaakWorker.cs
@@ -0,0 +1,27 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>
+/// Handles one acquired <c>OpenZaakAanmaken</c> external-worker job (ADR-0009): load the registration
+/// the job correlates to, open a zaak for it via the ACL (§8.1), attach the zaak to the aggregate, and
+/// return the zaak URL so the caller can complete the Flowable job. Pure application logic over ports —
+/// it knows nothing of Flowable; the polling loop that feeds it jobs lives in Infrastructure.
+/// </summary>
+public sealed class OpenZaakWorker(IRegistrationStore store, IAclClient acl)
+{
+    /// <summary>
+    /// Process the job and return the URL of the (existing or newly opened) zaak. Idempotent: if the
+    /// registration already has a zaak — a job redelivered after its completion was lost — it returns
+    /// that zaak without opening a second one (§8.6, at-least-once delivery). An unknown registration
+    /// is an error: it throws, leaving the job un-completed for Flowable to redeliver.
+    /// </summary>
+    public async Task<Uri> HandleAsync(OpenZaakJob job, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(job);
+
+        // STUB (red): does not load, call the ACL, or attach.
+        await Task.CompletedTask;
+        return new Uri("about:blank");
+    }
+}
diff --git a/services/domain/Big.Application/Ports.cs b/services/domain/Big.Application/Ports.cs
new file mode 100644
index 0000000..800b036
--- /dev/null
+++ b/services/domain/Big.Application/Ports.cs
@@ -0,0 +1,47 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>
+/// The port to the workflow engine. Implemented by the Workflow Client in Infrastructure — the
+/// <em>only</em> code that talks to Flowable (CLAUDE.md §8.2). The application asks it to start the
+/// registratie process; it never knows Flowable exists.
+/// </summary>
+public interface IWorkflowClient
+{
+    /// <summary>
+    /// Start one <c>registratie</c> process instance for the given registration, carrying the
+    /// registration id so the <c>OpenZaakAanmaken</c> external task can be correlated back to its
+    /// aggregate. Returns the process instance id.
+    /// </summary>
+    Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default);
+}
+
+/// <summary>
+/// The port to the Anti-Corruption Layer. Implemented in Infrastructure by an HTTP client to the
+/// ACL service — the <em>only</em> code that talks to ZGW (CLAUDE.md §8.1). The domain hands over a
+/// bsn; the ACL default-fills the ZGW-mandatory fields (ADR-0003) and returns the created zaak URL.
+/// </summary>
+public interface IAclClient
+{
+    Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default);
+}
+
+/// <summary>
+/// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice
+/// (ADR-0009); an EF-backed store is a documented follow-up, and this port keeps that change additive.
+/// </summary>
+public interface IRegistrationStore
+{
+    /// <summary>Insert or update the registration, keyed on its id.</summary>
+    Task SaveAsync(Registration registration, CancellationToken ct = default);
+
+    /// <summary>Load a registration by id, or <c>null</c> if none exists.</summary>
+    Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default);
+}
+
+/// <summary>
+/// An acquired <c>OpenZaakAanmaken</c> external-worker job: the Flowable job id (needed to complete
+/// it) and the registration id it carries as a process variable.
+/// </summary>
+public sealed record OpenZaakJob(string JobId, RegistrationId RegistrationId);
diff --git a/services/domain/Big.Application/SubmitRegistration.cs b/services/domain/Big.Application/SubmitRegistration.cs
new file mode 100644
index 0000000..c4f3555
--- /dev/null
+++ b/services/domain/Big.Application/SubmitRegistration.cs
@@ -0,0 +1,25 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A zorgprofessional's request to register, in domain language. No ZGW concepts.</summary>
+public sealed record SubmitRegistrationCommand(string Bsn);
+
+/// <summary>
+/// The submit use case: create the <see cref="Registration"/> aggregate (INGEDIEND), persist it,
+/// then start the registratie workflow process and record its instance id. It returns as soon as
+/// the process is started — opening the zaak happens later, off the request path, in the external-task
+/// worker (ADR-0009). Persisting <em>before</em> starting the process closes the race where the worker
+/// acquires the OpenZaakAanmaken job before the aggregate it correlates to exists.
+/// </summary>
+public sealed class SubmitRegistration(IRegistrationStore store, IWorkflowClient workflow)
+{
+    public async Task<RegistrationId> HandleAsync(SubmitRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        // STUB (red): no persistence, no process start.
+        await Task.CompletedTask;
+        return RegistrationId.New();
+    }
+}
diff --git a/services/domain/Big.Tests/Big.Tests.csproj b/services/domain/Big.Tests/Big.Tests.csproj
index 575ce15..52fba41 100644
--- a/services/domain/Big.Tests/Big.Tests.csproj
+++ b/services/domain/Big.Tests/Big.Tests.csproj
@@ -20,6 +20,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\Big.Domain\Big.Domain.csproj" />
+    <ProjectReference Include="..\Big.Application\Big.Application.csproj" />
   </ItemGroup>
 
 </Project>
diff --git a/services/domain/Big.Tests/Fakes.cs b/services/domain/Big.Tests/Fakes.cs
new file mode 100644
index 0000000..d32397f
--- /dev/null
+++ b/services/domain/Big.Tests/Fakes.cs
@@ -0,0 +1,60 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+/// <summary>An in-memory <see cref="IRegistrationStore"/> for the application-layer tests. Upserts
+/// keyed on the registration id, mirroring the production in-memory store.</summary>
+internal sealed class InMemoryRegistrationStore : IRegistrationStore
+{
+    private readonly Dictionary<RegistrationId, Registration> _byId = [];
+
+    public int SaveCount { get; private set; }
+
+    public Task SaveAsync(Registration registration, CancellationToken ct = default)
+    {
+        SaveCount++;
+        _byId[registration.Id] = registration;
+        return Task.CompletedTask;
+    }
+
+    public Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default)
+        => Task.FromResult(_byId.GetValueOrDefault(id));
+
+    public void Seed(Registration registration) => _byId[registration.Id] = registration;
+}
+
+/// <summary>A fake Workflow Client that records the registration it was asked to start a process for
+/// and returns a fixed process-instance id. An optional callback runs at start time, letting a test
+/// assert ordering (e.g. that the registration was persisted before the process started).</summary>
+internal sealed class FakeWorkflowClient(string processInstanceId = "proc-1", Action<RegistrationId>? onStart = null)
+    : IWorkflowClient
+{
+    public RegistrationId? StartedFor { get; private set; }
+
+    public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    {
+        onStart?.Invoke(registrationId);
+        StartedFor = registrationId;
+        return Task.FromResult(processInstanceId);
+    }
+}
+
+/// <summary>A fake ACL client that records the bsn it was asked to open a zaak for and returns a
+/// fixed zaak URL.</summary>
+internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
+{
+    public static readonly Uri DefaultZaakUrl = new("http://openzaak/zaken/api/v1/zaken/abc");
+
+    private readonly Uri _zaakUrl = zaakUrl ?? DefaultZaakUrl;
+
+    public string? OpenedForBsn { get; private set; }
+    public int CallCount { get; private set; }
+
+    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    {
+        CallCount++;
+        OpenedForBsn = bsn;
+        return Task.FromResult(_zaakUrl);
+    }
+}
diff --git a/services/domain/Big.Tests/OpenZaakWorkerTests.cs b/services/domain/Big.Tests/OpenZaakWorkerTests.cs
new file mode 100644
index 0000000..c00ba7a
--- /dev/null
+++ b/services/domain/Big.Tests/OpenZaakWorkerTests.cs
@@ -0,0 +1,60 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class OpenZaakWorkerTests
+{
+    private static Registration Submitted(string bsn = "123456782")
+    {
+        var registration = Registration.Submit(bsn);
+        registration.RecordProcessStarted("proc-1");
+        return registration;
+    }
+
+    [Fact]
+    public async Task Handling_a_job_opens_a_zaak_via_the_acl_and_attaches_it()
+    {
+        var registration = Submitted();
+        var store = new InMemoryRegistrationStore();
+        store.Seed(registration);
+        var acl = new FakeAclClient();
+        var worker = new OpenZaakWorker(store, acl);
+
+        var zaakUrl = await worker.HandleAsync(new OpenZaakJob("job-1", registration.Id));
+
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, zaakUrl);
+        Assert.Equal("123456782", acl.OpenedForBsn);
+        var saved = await store.GetAsync(registration.Id);
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, saved!.ZaakUrl);
+    }
+
+    [Fact]
+    public async Task Handling_a_job_for_an_unknown_registration_throws_and_opens_no_zaak()
+    {
+        var store = new InMemoryRegistrationStore();
+        var acl = new FakeAclClient();
+        var worker = new OpenZaakWorker(store, acl);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => worker.HandleAsync(new OpenZaakJob("job-1", RegistrationId.New())));
+        Assert.Equal(0, acl.CallCount);
+    }
+
+    [Fact]
+    public async Task Handling_a_redelivered_job_is_idempotent_and_does_not_open_a_second_zaak()
+    {
+        var registration = Submitted();
+        var store = new InMemoryRegistrationStore();
+        store.Seed(registration);
+        var acl = new FakeAclClient();
+        var worker = new OpenZaakWorker(store, acl);
+        var job = new OpenZaakJob("job-1", registration.Id);
+
+        var first = await worker.HandleAsync(job);
+        var second = await worker.HandleAsync(job);
+
+        Assert.Equal(first, second);
+        Assert.Equal(1, acl.CallCount);
+    }
+}
diff --git a/services/domain/Big.Tests/SubmitRegistrationTests.cs b/services/domain/Big.Tests/SubmitRegistrationTests.cs
new file mode 100644
index 0000000..b1b15a7
--- /dev/null
+++ b/services/domain/Big.Tests/SubmitRegistrationTests.cs
@@ -0,0 +1,52 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class SubmitRegistrationTests
+{
+    [Fact]
+    public async Task Submitting_persists_an_ingediend_registration_and_starts_the_process()
+    {
+        var store = new InMemoryRegistrationStore();
+        var workflow = new FakeWorkflowClient(processInstanceId: "proc-42");
+        var handler = new SubmitRegistration(store, workflow);
+
+        var id = await handler.HandleAsync(new SubmitRegistrationCommand("123456782"));
+
+        var saved = await store.GetAsync(id);
+        Assert.NotNull(saved);
+        Assert.Equal("123456782", saved.Bsn);
+        Assert.Equal(RegistrationStatus.Ingediend, saved.Status);
+        Assert.Equal("proc-42", saved.ProcessInstanceId);
+        Assert.Equal(id, workflow.StartedFor);
+        Assert.Null(saved.ZaakUrl);
+    }
+
+    [Fact]
+    public async Task Submitting_persists_the_registration_before_starting_the_process()
+    {
+        // The worker correlates the OpenZaakAanmaken job back to the aggregate by id, so the
+        // aggregate must already be persisted when the process starts (ADR-0009).
+        var store = new InMemoryRegistrationStore();
+        Registration? visibleAtStart = null;
+        var workflow = new FakeWorkflowClient(onStart: id => visibleAtStart = store.GetAsync(id).Result);
+        var handler = new SubmitRegistration(store, workflow);
+
+        await handler.HandleAsync(new SubmitRegistrationCommand("123456782"));
+
+        Assert.NotNull(visibleAtStart);
+    }
+
+    [Fact]
+    public async Task Submitting_without_a_bsn_is_rejected_and_starts_no_process()
+    {
+        var store = new InMemoryRegistrationStore();
+        var workflow = new FakeWorkflowClient();
+        var handler = new SubmitRegistration(store, workflow);
+
+        await Assert.ThrowsAnyAsync<ArgumentException>(
+            () => handler.HandleAsync(new SubmitRegistrationCommand("")));
+        Assert.Null(workflow.StartedFor);
+    }
+}
diff --git a/services/domain/Big.slnx b/services/domain/Big.slnx
index 6125bff..78b7664 100644
--- a/services/domain/Big.slnx
+++ b/services/domain/Big.slnx
@@ -1,4 +1,5 @@
 <Solution>
   <Project Path="Big.Domain/Big.Domain.csproj" />
+  <Project Path="Big.Application/Big.Application.csproj" />
   <Project Path="Big.Tests/Big.Tests.csproj" />
 </Solution>