diff --git a/SharepointToolbox.Tests/Services/GraphUserDirectoryServiceTests.cs b/SharepointToolbox.Tests/Services/GraphUserDirectoryServiceTests.cs
new file mode 100644
index 0000000..bfdf141
--- /dev/null
+++ b/SharepointToolbox.Tests/Services/GraphUserDirectoryServiceTests.cs
@@ -0,0 +1,150 @@
+using Microsoft.Graph.Models;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.Tests.Services;
+
+/// <summary>
+/// Unit tests for <see cref="GraphUserDirectoryService"/> (Phase 10 Plan 02).
+///
+/// Testing strategy: GraphUserDirectoryService wraps Microsoft Graph SDK's PageIterator,
+/// whose constructor is internal and cannot be mocked without a real GraphServiceClient.
+/// Full pagination/cancellation tests therefore require integration-level setup.
+///
+/// We test what IS unit-testable:
+///   1. MapUser — the static mapping method that converts a Graph User to GraphDirectoryUser.
+///      This covers all 5 required fields and the DisplayName fallback logic.
+///   2. GetUsersAsync integration paths are documented with Skip tests that explain the
+///      constraint and serve as living documentation of intended behaviour.
+/// </summary>
+[Trait("Category", "Unit")]
+public class GraphUserDirectoryServiceTests
+{
+    // ── MapUser: field mapping ────────────────────────────────────────────────
+
+    [Fact]
+    public void MapUser_AllFieldsPresent_MapsCorrectly()
+    {
+        var user = new User
+        {
+            DisplayName = "Alice Smith",
+            UserPrincipalName = "alice@contoso.com",
+            Mail = "alice@contoso.com",
+            Department = "Engineering",
+            JobTitle = "Senior Developer"
+        };
+
+        var result = GraphUserDirectoryService.MapUser(user);
+
+        Assert.Equal("Alice Smith", result.DisplayName);
+        Assert.Equal("alice@contoso.com", result.UserPrincipalName);
+        Assert.Equal("alice@contoso.com", result.Mail);
+        Assert.Equal("Engineering", result.Department);
+        Assert.Equal("Senior Developer", result.JobTitle);
+    }
+
+    [Fact]
+    public void MapUser_NullDisplayName_FallsBackToUserPrincipalName()
+    {
+        var user = new User
+        {
+            DisplayName = null,
+            UserPrincipalName = "bob@contoso.com",
+            Mail = null,
+            Department = null,
+            JobTitle = null
+        };
+
+        var result = GraphUserDirectoryService.MapUser(user);
+
+        Assert.Equal("bob@contoso.com", result.DisplayName);
+        Assert.Equal("bob@contoso.com", result.UserPrincipalName);
+        Assert.Null(result.Mail);
+        Assert.Null(result.Department);
+        Assert.Null(result.JobTitle);
+    }
+
+    [Fact]
+    public void MapUser_NullDisplayNameAndNullUPN_FallsBackToEmptyString()
+    {
+        var user = new User
+        {
+            DisplayName = null,
+            UserPrincipalName = null,
+            Mail = null,
+            Department = null,
+            JobTitle = null
+        };
+
+        var result = GraphUserDirectoryService.MapUser(user);
+
+        Assert.Equal(string.Empty, result.DisplayName);
+        Assert.Equal(string.Empty, result.UserPrincipalName);
+    }
+
+    [Fact]
+    public void MapUser_NullUPN_ReturnsEmptyStringForUPN()
+    {
+        var user = new User
+        {
+            DisplayName = "Carol Jones",
+            UserPrincipalName = null,
+            Mail = "carol@contoso.com",
+            Department = "Marketing",
+            JobTitle = "Manager"
+        };
+
+        var result = GraphUserDirectoryService.MapUser(user);
+
+        Assert.Equal("Carol Jones", result.DisplayName);
+        Assert.Equal(string.Empty, result.UserPrincipalName);
+        Assert.Equal("carol@contoso.com", result.Mail);
+    }
+
+    [Fact]
+    public void MapUser_OptionalFieldsNull_ProducesNullableNullProperties()
+    {
+        var user = new User
+        {
+            DisplayName = "Dave Brown",
+            UserPrincipalName = "dave@contoso.com",
+            Mail = null,
+            Department = null,
+            JobTitle = null
+        };
+
+        var result = GraphUserDirectoryService.MapUser(user);
+
+        Assert.Null(result.Mail);
+        Assert.Null(result.Department);
+        Assert.Null(result.JobTitle);
+    }
+
+    // ── GetUsersAsync: integration-level scenarios (skipped without live tenant) ──
+
+    [Fact(Skip = "Requires integration test with real Graph client — PageIterator.CreatePageIterator " +
+                 "uses internal GraphServiceClient request execution that cannot be mocked via Moq. " +
+                 "Intended behaviour: returns all users matching filter across all pages, " +
+                 "correctly mapping all 5 fields per user.")]
+    public Task GetUsersAsync_SinglePage_ReturnsMappedUsers()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires integration test with real Graph client. " +
+                 "Intended behaviour: IProgress<int>.Report is called once per user " +
+                 "with an incrementing count (1, 2, 3, ...).")]
+    public Task GetUsersAsync_ReportsProgressWithIncrementingCount()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires integration test with real Graph client. " +
+                 "Intended behaviour: when CancellationToken is cancelled during iteration, " +
+                 "the callback returns false and iteration stops, returning partial results " +
+                 "(or OperationCanceledException if cancellation fires before first page).")]
+    public Task GetUsersAsync_CancelledToken_StopsIteration()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires integration test with real Graph client. " +
+                 "Intended behaviour: when Graph returns null response, " +
+                 "GetUsersAsync returns an empty IReadOnlyList without throwing.")]
+    public Task GetUsersAsync_NullResponse_ReturnsEmptyList()
+        => Task.CompletedTask;
+}
diff --git a/SharepointToolbox/Services/GraphUserDirectoryService.cs b/SharepointToolbox/Services/GraphUserDirectoryService.cs
new file mode 100644
index 0000000..63772b7
--- /dev/null
+++ b/SharepointToolbox/Services/GraphUserDirectoryService.cs
@@ -0,0 +1,78 @@
+using Microsoft.Graph;
+using Microsoft.Graph.Models;
+using SharepointToolbox.Core.Models;
+using AppGraphClientFactory = SharepointToolbox.Infrastructure.Auth.GraphClientFactory;
+
+namespace SharepointToolbox.Services;
+
+/// <summary>
+/// Enumerates all enabled member users from a tenant via Microsoft Graph,
+/// using PageIterator for transparent multi-page iteration.
+/// Used by Phase 13's User Directory ViewModel.
+/// </summary>
+public class GraphUserDirectoryService : IGraphUserDirectoryService
+{
+    private readonly AppGraphClientFactory _graphClientFactory;
+
+    public GraphUserDirectoryService(AppGraphClientFactory graphClientFactory)
+    {
+        _graphClientFactory = graphClientFactory;
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<GraphDirectoryUser>> GetUsersAsync(
+        string clientId,
+        IProgress<int>? progress = null,
+        CancellationToken ct = default)
+    {
+        var graphClient = await _graphClientFactory.CreateClientAsync(clientId, ct);
+
+        var response = await graphClient.Users.GetAsync(config =>
+        {
+            // Pending real-tenant verification — see STATE.md pending todos
+            config.QueryParameters.Filter = "accountEnabled eq true and userType eq 'Member'";
+            config.QueryParameters.Select = new[]
+            {
+                "displayName", "userPrincipalName", "mail", "department", "jobTitle"
+            };
+            config.QueryParameters.Top = 999;
+            // No ConsistencyLevel header: standard equality filter does not require eventual consistency
+        }, ct);
+
+        if (response is null)
+            return Array.Empty<GraphDirectoryUser>();
+
+        var results = new List<GraphDirectoryUser>();
+
+        var pageIterator = PageIterator<User, UserCollectionResponse>.CreatePageIterator(
+            graphClient,
+            response,
+            user =>
+            {
+                // Honour cancellation inside the callback — returning false stops iteration
+                if (ct.IsCancellationRequested)
+                    return false;
+
+                results.Add(MapUser(user));
+                progress?.Report(results.Count);
+                return true;
+            });
+
+        await pageIterator.IterateAsync(ct);
+
+        return results;
+    }
+
+    /// <summary>
+    /// Maps a Graph SDK <see cref="User"/> object to a <see cref="GraphDirectoryUser"/> record.
+    /// Extracted as an internal static method to allow direct unit-test coverage of mapping logic
+    /// without requiring a live Graph endpoint.
+    /// </summary>
+    internal static GraphDirectoryUser MapUser(User user) =>
+        new(
+            DisplayName: user.DisplayName ?? user.UserPrincipalName ?? string.Empty,
+            UserPrincipalName: user.UserPrincipalName ?? string.Empty,
+            Mail: user.Mail,
+            Department: user.Department,
+            JobTitle: user.JobTitle);
+}