diff --git a/src/TeamsISO.Engine/Discovery/ParticipantTracker.cs b/src/TeamsISO.Engine/Discovery/ParticipantTracker.cs
new file mode 100644
index 0000000..3b94340
--- /dev/null
+++ b/src/TeamsISO.Engine/Discovery/ParticipantTracker.cs
@@ -0,0 +1,104 @@
+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Discovery;
+
+/// <summary>
+/// Maintains the operator-facing participant list, applying the rename heuristic
+/// from the v1 spec: if a participant source disappears and another participant
+/// source with the same MachineName appears within <see cref="_renameWindow"/>,
+/// the existing <see cref="Participant.Id"/> transfers to the new source.
+/// </summary>
+public sealed class ParticipantTracker
+{
+    private readonly TimeSpan _renameWindow;
+    private readonly Func<DateTimeOffset> _now;
+    private readonly List<MutableParticipant> _participants = new();
+    private readonly List<RecentlyRemoved> _recentlyRemoved = new();
+
+    public ParticipantTracker(TimeSpan renameWindow, Func<DateTimeOffset> now)
+    {
+        _renameWindow = renameWindow;
+        _now = now;
+    }
+
+    public IReadOnlyList<Participant> Participants =>
+        _participants.Select(m => m.ToRecord()).ToList();
+
+    public void Apply(DiscoveryEvent ev)
+    {
+        switch (ev)
+        {
+            case DiscoveryEvent.Added a when a.Source.Kind == NdiSourceKind.Participant:
+                HandleAdded(a.Source);
+                break;
+            case DiscoveryEvent.Removed r when r.Source.Kind == NdiSourceKind.Participant:
+                HandleRemoved(r.Source);
+                break;
+        }
+    }
+
+    private void HandleAdded(NdiSource source)
+    {
+        var now = _now();
+        PruneRecentlyRemoved(now);
+
+        var match = _recentlyRemoved.FirstOrDefault(rr => rr.MachineName == source.MachineName);
+        if (match is not null)
+        {
+            var existing = _participants.First(p => p.Id == match.Id);
+            existing.DisplayName = source.DisplayName!;
+            existing.CurrentSource = source;
+            existing.LastSeen = now;
+            _recentlyRemoved.Remove(match);
+            return;
+        }
+
+        _participants.Add(new MutableParticipant(
+            Id: Guid.NewGuid(),
+            DisplayName: source.DisplayName!,
+            CurrentSource: source,
+            FirstSeen: now,
+            LastSeen: now));
+    }
+
+    private void HandleRemoved(NdiSource source)
+    {
+        var now = _now();
+        var existing = _participants.FirstOrDefault(p =>
+            p.CurrentSource is not null && p.CurrentSource.FullName == source.FullName);
+        if (existing is null)
+            return;
+
+        existing.CurrentSource = null;
+        _recentlyRemoved.Add(new RecentlyRemoved(existing.Id, source.MachineName, now));
+    }
+
+    private void PruneRecentlyRemoved(DateTimeOffset now)
+    {
+        _recentlyRemoved.RemoveAll(rr => now - rr.RemovedAt > _renameWindow);
+    }
+
+    private sealed class MutableParticipant
+    {
+        public Guid Id { get; init; }
+        public string DisplayName { get; set; }
+        public NdiSource? CurrentSource { get; set; }
+        public DateTimeOffset FirstSeen { get; init; }
+        public DateTimeOffset LastSeen { get; set; }
+
+        public MutableParticipant(Guid Id, string DisplayName, NdiSource? CurrentSource,
+            DateTimeOffset FirstSeen, DateTimeOffset LastSeen)
+        {
+            this.Id = Id;
+            this.DisplayName = DisplayName;
+            this.CurrentSource = CurrentSource;
+            this.FirstSeen = FirstSeen;
+            this.LastSeen = LastSeen;
+        }
+
+        public Participant ToRecord() =>
+            new(Id, DisplayName, CurrentSource, FirstSeen, LastSeen);
+    }
+
+    private sealed record RecentlyRemoved(Guid Id, string MachineName, DateTimeOffset RemovedAt);
+}
diff --git a/src/tests/TeamsISO.Engine.Tests/Discovery/ParticipantTrackerTests.cs b/src/tests/TeamsISO.Engine.Tests/Discovery/ParticipantTrackerTests.cs
new file mode 100644
index 0000000..0892fab
--- /dev/null
+++ b/src/tests/TeamsISO.Engine.Tests/Discovery/ParticipantTrackerTests.cs
@@ -0,0 +1,103 @@
+using TeamsISO.Engine.Discovery;
+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Tests.Discovery;
+
+public class ParticipantTrackerTests
+{
+    private static NdiSource Source(string machine, string display) =>
+        new($"{machine} (Teams - {display})", machine, NdiSourceKind.Participant, display);
+
+    private static DateTimeOffset T0 => DateTimeOffset.UnixEpoch;
+
+    [Fact]
+    public void Added_CreatesParticipant()
+    {
+        var tracker = new ParticipantTracker(renameWindow: TimeSpan.FromSeconds(5), now: () => T0);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane")));
+
+        tracker.Participants.Should().HaveCount(1);
+        var p = tracker.Participants[0];
+        p.DisplayName.Should().Be("Jane");
+        p.CurrentSource!.MachineName.Should().Be("PC1");
+    }
+
+    [Fact]
+    public void Removed_NullsCurrentSourceButKeepsParticipant()
+    {
+        var time = T0;
+        var tracker = new ParticipantTracker(TimeSpan.FromSeconds(5), () => time);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane")));
+
+        time = T0.AddSeconds(1);
+        tracker.Apply(new DiscoveryEvent.Removed(Source("PC1", "Jane")));
+
+        tracker.Participants.Should().HaveCount(1);
+        tracker.Participants[0].CurrentSource.Should().BeNull();
+    }
+
+    [Fact]
+    public void RenameWithinWindow_TransfersParticipantId()
+    {
+        var time = T0;
+        var tracker = new ParticipantTracker(TimeSpan.FromSeconds(5), () => time);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane")));
+        var originalId = tracker.Participants[0].Id;
+
+        time = T0.AddSeconds(1);
+        tracker.Apply(new DiscoveryEvent.Removed(Source("PC1", "Jane")));
+
+        time = T0.AddSeconds(3);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane (PM)")));
+
+        tracker.Participants.Should().HaveCount(1);
+        tracker.Participants[0].Id.Should().Be(originalId);
+        tracker.Participants[0].DisplayName.Should().Be("Jane (PM)");
+    }
+
+    [Fact]
+    public void RenameAfterWindow_TreatsAsNewParticipant()
+    {
+        var time = T0;
+        var tracker = new ParticipantTracker(TimeSpan.FromSeconds(5), () => time);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane")));
+        var originalId = tracker.Participants[0].Id;
+
+        time = T0.AddSeconds(1);
+        tracker.Apply(new DiscoveryEvent.Removed(Source("PC1", "Jane")));
+
+        time = T0.AddSeconds(20);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Bob")));
+
+        tracker.Participants.Should().HaveCount(2);
+        tracker.Participants.Should().Contain(p => p.Id == originalId);
+        tracker.Participants.Should().Contain(p => p.DisplayName == "Bob" && p.Id != originalId);
+    }
+
+    [Fact]
+    public void DifferentMachine_DoesNotInheritIdentity()
+    {
+        var time = T0;
+        var tracker = new ParticipantTracker(TimeSpan.FromSeconds(5), () => time);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC1", "Jane")));
+
+        time = T0.AddSeconds(1);
+        tracker.Apply(new DiscoveryEvent.Removed(Source("PC1", "Jane")));
+
+        time = T0.AddSeconds(2);
+        tracker.Apply(new DiscoveryEvent.Added(Source("PC2", "Jane (PM)")));
+
+        tracker.Participants.Should().HaveCount(2);
+    }
+
+    [Fact]
+    public void NonParticipantSources_AreIgnored()
+    {
+        var tracker = new ParticipantTracker(TimeSpan.FromSeconds(5), () => T0);
+        var screen = new NdiSource("PC1 (Teams Screen Share)", "PC1", NdiSourceKind.ScreenShare, null);
+
+        tracker.Apply(new DiscoveryEvent.Added(screen));
+
+        tracker.Participants.Should().BeEmpty();
+    }
+}