diff --git a/src/TeamsISO.Engine/Domain/EngineAlert.cs b/src/TeamsISO.Engine/Domain/EngineAlert.cs
new file mode 100644
index 0000000..b9189cd
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/EngineAlert.cs
@@ -0,0 +1,19 @@
+namespace TeamsISO.Engine.Domain;
+
+/// <summary>
+/// Structured engine alerts for UI banner display and ops logging.
+/// </summary>
+public abstract record EngineAlert(string Message)
+{
+    public sealed record NdiRuntimeMismatch(string DetectedVersion, string ExpectedVersion)
+        : EngineAlert($"NDI runtime version mismatch: detected {DetectedVersion}, expected {ExpectedVersion}.");
+
+    public sealed record OutputNameCollision(string Name)
+        : EngineAlert($"Another TeamsISO instance on the LAN is emitting an output named '{Name}'.");
+
+    public sealed record PipelineError(Guid ParticipantId, string Reason)
+        : EngineAlert($"Pipeline {ParticipantId} entered Error: {Reason}");
+
+    public sealed record ConfigSaveFailed(string Reason)
+        : EngineAlert($"Failed to save configuration: {Reason}");
+}
diff --git a/src/TeamsISO.Engine/Domain/EngineConfig.cs b/src/TeamsISO.Engine/Domain/EngineConfig.cs
new file mode 100644
index 0000000..6b37927
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/EngineConfig.cs
@@ -0,0 +1,9 @@
+namespace TeamsISO.Engine.Domain;
+
+public sealed record EngineConfig(
+    FrameProcessingSettings Global,
+    IReadOnlyList<IsoAssignment> Assignments)
+{
+    public static readonly EngineConfig Default =
+        new(FrameProcessingSettings.Default, Array.Empty<IsoAssignment>());
+}
diff --git a/src/TeamsISO.Engine/Domain/FrameProcessingSettings.cs b/src/TeamsISO.Engine/Domain/FrameProcessingSettings.cs
new file mode 100644
index 0000000..74c3cc8
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/FrameProcessingSettings.cs
@@ -0,0 +1,34 @@
+namespace TeamsISO.Engine.Domain;
+
+public sealed record FrameProcessingSettings(
+    TargetFramerate Framerate,
+    TargetResolution Resolution,
+    AspectMode Aspect,
+    AudioMode Audio)
+{
+    public static readonly FrameProcessingSettings Default =
+        new(TargetFramerate.Fps30, TargetResolution.R1080p, AspectMode.Pillarbox, AudioMode.Auto);
+
+    /// <summary>Returns the framerate enum value as a numeric frames-per-second.</summary>
+    public double FramerateHz => Framerate switch
+    {
+        TargetFramerate.Fps23_976 => 24000.0 / 1001.0,
+        TargetFramerate.Fps24     => 24.0,
+        TargetFramerate.Fps25     => 25.0,
+        TargetFramerate.Fps29_97  => 30000.0 / 1001.0,
+        TargetFramerate.Fps30     => 30.0,
+        TargetFramerate.Fps50     => 50.0,
+        TargetFramerate.Fps59_94  => 60000.0 / 1001.0,
+        TargetFramerate.Fps60     => 60.0,
+        _ => throw new InvalidOperationException($"Unknown framerate: {Framerate}")
+    };
+
+    /// <summary>Returns the resolution as (width, height).</summary>
+    public (int Width, int Height) ResolutionSize => Resolution switch
+    {
+        TargetResolution.R720p  => (1280, 720),
+        TargetResolution.R1080p => (1920, 1080),
+        TargetResolution.R4K    => (3840, 2160),
+        _ => throw new InvalidOperationException($"Unknown resolution: {Resolution}")
+    };
+}
diff --git a/src/TeamsISO.Engine/Domain/IsoAssignment.cs b/src/TeamsISO.Engine/Domain/IsoAssignment.cs
new file mode 100644
index 0000000..a62e0cd
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/IsoAssignment.cs
@@ -0,0 +1,9 @@
+namespace TeamsISO.Engine.Domain;
+
+/// <summary>
+/// Operator's intent for an ISO output. Persisted to <c>config.json</c>.
+/// </summary>
+public sealed record IsoAssignment(
+    Guid ParticipantId,
+    bool IsEnabled,
+    string? CustomOutputName);
diff --git a/src/TeamsISO.Engine/Domain/IsoHealthStats.cs b/src/TeamsISO.Engine/Domain/IsoHealthStats.cs
new file mode 100644
index 0000000..84130a0
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/IsoHealthStats.cs
@@ -0,0 +1,15 @@
+namespace TeamsISO.Engine.Domain;
+
+public sealed record IsoHealthStats(
+    long FramesIn,
+    long FramesOut,
+    long FramesDropped,
+    long FramesDuplicated,
+    DateTimeOffset? LastFrameAt,
+    double IncomingFps,
+    int IncomingWidth,
+    int IncomingHeight)
+{
+    public static readonly IsoHealthStats Empty =
+        new(0, 0, 0, 0, null, 0, 0, 0);
+}
diff --git a/src/TeamsISO.Engine/Domain/IsoOutput.cs b/src/TeamsISO.Engine/Domain/IsoOutput.cs
new file mode 100644
index 0000000..4a660f4
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/IsoOutput.cs
@@ -0,0 +1,7 @@
+namespace TeamsISO.Engine.Domain;
+
+public sealed record IsoOutput(
+    Guid ParticipantId,
+    string EffectiveOutputName,
+    IsoHealthStats Stats,
+    IsoState State);
diff --git a/src/TeamsISO.Engine/Domain/Participant.cs b/src/TeamsISO.Engine/Domain/Participant.cs
new file mode 100644
index 0000000..2a38b67
--- /dev/null
+++ b/src/TeamsISO.Engine/Domain/Participant.cs
@@ -0,0 +1,12 @@
+namespace TeamsISO.Engine.Domain;
+
+/// <summary>
+/// Operator-facing identity for a single human in the meeting.
+/// <c>Id</c> is engine-assigned and stable across the rename heuristic.
+/// </summary>
+public sealed record Participant(
+    Guid Id,
+    string DisplayName,
+    NdiSource? CurrentSource,
+    DateTimeOffset FirstSeen,
+    DateTimeOffset LastSeen);