diff --git a/src/gaemstone.SourceGen/ModuleGenerator.cs b/src/gaemstone.SourceGen/ModuleGenerator.cs
index 3a83f0a..3049e2c 100644
--- a/src/gaemstone.SourceGen/ModuleGenerator.cs
+++ b/src/gaemstone.SourceGen/ModuleGenerator.cs
@@ -242,7 +242,7 @@ public class ModuleGenerator
 
 							sb.Append(p.Kind switch {
 								ParameterKind.Has => ".None",
-								ParameterKind.Not => ".None.Not",
+								ParameterKind.Not => ".Not",
 								ParameterKind.Ref => ".InOut",
 								ParameterKind.Out => ".Out",
 								_ when !p.IsValueType => ".InOut", // Reference types always imply writability.
@@ -289,6 +289,10 @@ public class ModuleGenerator
 				sb.AppendLine($"\t\t{@var}.Add<{a.GetFullName()}>();");
 			foreach (var (r, t) in e.RelationsToAdd)
 				sb.AppendLine($"\t\t{@var}.Add<{r.GetFullName()}, {t.GetFullName()}>();");
+
+			// If system doesn't have an explicit phase set, default to OnUpdate.
+			if (e is MethodEntityInfo { IsSystem: true, HasPhaseSet: false })
+				sb.AppendLine($"\t\t{@var}.Add<gaemstone.Flecs.Core.DependsOn, gaemstone.Flecs.Pipeline.OnUpdate>();");
 		}
 	}
 
diff --git a/src/gaemstone.SourceGen/Structure/BaseEntityInfo.cs b/src/gaemstone.SourceGen/Structure/BaseEntityInfo.cs
index 8c62dab..5add683 100644
--- a/src/gaemstone.SourceGen/Structure/BaseEntityInfo.cs
+++ b/src/gaemstone.SourceGen/Structure/BaseEntityInfo.cs
@@ -13,8 +13,8 @@ public abstract class BaseEntityInfo : BaseInfo
 	public string? EntitySymbol { get; }
 
 	public List<INamedTypeSymbol> EntitiesToAdd { get; } = new();
-	public List<(INamedTypeSymbol, INamedTypeSymbol)> RelationsToAdd { get; } = new();
-	public bool HasEntitiesToAdd => (EntitiesToAdd.Count > 0) || (RelationsToAdd.Count > 0);
+	public List<(INamedTypeSymbol Relation, INamedTypeSymbol Target)> RelationsToAdd { get; } = new();
+	public virtual bool HasEntitiesToAdd => (EntitiesToAdd.Count > 0) || (RelationsToAdd.Count > 0);
 
 	public BaseEntityInfo(ISymbol symbol)
 		: base(symbol)
diff --git a/src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs b/src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs
index 8728fc1..8fb5c6f 100644
--- a/src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs
+++ b/src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs
@@ -22,6 +22,9 @@ public class MethodEntityInfo : BaseEntityInfo
 	public bool IsStatic  => Symbol.IsStatic;
 	public bool IsGeneric => Symbol.IsGenericMethod;
 
+	public bool HasPhaseSet { get; private set; }
+	public override bool HasEntitiesToAdd => base.HasEntitiesToAdd || !HasPhaseSet;
+
 	public MethodEntityInfo(ISymbol symbol)
 		: base(symbol)
 	{
@@ -79,6 +82,11 @@ public class MethodEntityInfo : BaseEntityInfo
 			if (param.HasTerm)
 				param.TermIndex = termIndex++;
 
+		// See if we have any [DependsOn<...>] attributes for this system.
+		// If not, ModuleGenerator will add [DependsOn<Flecs.Pipeline.OnUpdate>].
+		HasPhaseSet = IsSystem && RelationsToAdd.Any(entry => entry.Relation
+			.GetFullName(FullNameStyle.NoGeneric) == "gaemstone.ECS.DependsOnAttribute");
+
 		// TODO: Handle systems with [Source].
 		// TODO: Validate ObserverEvents.
 	}