diff --git a/OECS/OrderedEntitySet.cs b/OECS/OrderedEntitySet.cs
new file mode 100644
index 0000000..9e0d5a8
--- /dev/null
+++ b/OECS/OrderedEntitySet.cs
@@ -0,0 +1,49 @@
+using System.Collections;
+
+namespace OECS;
+
+/// <summary>
+/// An insertion-ordered set of entities that supports O(1) Add, Remove, and Contains
+/// while iterating in insertion order. Drop-in for cases where both
+/// stable ordering and fast membership checks are needed.
+///
+/// Not thread-safe.
+/// </summary>
+internal sealed class OrderedEntitySet : IReadOnlyCollection<Entity>
+{
+    // Entities are stored in a packed list; a secondary dictionary maps
+    // each entity to its index for O(1) removal via swap-with-last.
+    private readonly List<Entity> _order = new();
+    private readonly Dictionary<Entity, int> _index = new();
+
+    public int Count => _order.Count;
+
+    public bool Add(Entity entity)
+    {
+        if (_index.ContainsKey(entity))
+            return false;
+
+        _index[entity] = _order.Count;
+        _order.Add(entity);
+        return true;
+    }
+
+    public bool Remove(Entity entity)
+    {
+        if (!_index.Remove(entity, out var idx))
+            return false;
+
+        int lastIdx = _order.Count - 1;
+        if (idx < lastIdx)
+        {
+            var last = _order[lastIdx];
+            _order[idx] = last;
+            _index[last] = idx;
+        }
+        _order.RemoveAt(lastIdx);
+        return true;
+    }
+
+    public IEnumerator<Entity> GetEnumerator() => _order.GetEnumerator();
+    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+}
\ No newline at end of file
diff --git a/OECS/RelationshipIndex.cs b/OECS/RelationshipIndex.cs
index 1436da4..f5d1572 100644
--- a/OECS/RelationshipIndex.cs
+++ b/OECS/RelationshipIndex.cs
@@ -8,13 +8,16 @@ namespace OECS;
 ///
 /// Updated automatically by <see cref="World"/> when relationship
 /// components are added, removed, or when entities are destroyed.
+///
+/// Source sets are insertion-ordered — the order in which relationships
+/// are added is preserved during iteration.
 /// </summary>
 internal class RelationshipIndex
 {
     /// <summary>
     /// Per relationship type: target entity → set of source entities.
     /// </summary>
-    private readonly Dictionary<Type, Dictionary<Entity, HashSet<Entity>>> _index = new();
+    private readonly Dictionary<Type, Dictionary<Entity, OrderedEntitySet>> _index = new();
 
     /// <summary>
     /// Registers that a source entity now has a relationship pointing to a target.
@@ -23,13 +26,13 @@ internal class RelationshipIndex
     {
         if (!_index.TryGetValue(relationshipType, out var targetMap))
         {
-            targetMap = new Dictionary<Entity, HashSet<Entity>>();
+            targetMap = new Dictionary<Entity, OrderedEntitySet>();
             _index[relationshipType] = targetMap;
         }
 
         if (!targetMap.TryGetValue(target, out var sources))
         {
-            sources = new HashSet<Entity>();
+            sources = new OrderedEntitySet();
             targetMap[target] = sources;
         }
 
@@ -55,7 +58,7 @@ internal class RelationshipIndex
 
     /// <summary>
     /// Returns all source entities that have a relationship of the given type
-    /// pointing to the specified target entity.
+    /// pointing to the specified target entity, in insertion order.
     /// </summary>
     public IReadOnlyCollection<Entity> GetSources<T>(Entity target)
         where T : struct, IRelationship
@@ -123,4 +126,4 @@ internal class RelationshipIndex
     {
         return _index.ContainsKey(relationshipType);
     }
-}
+}
\ No newline at end of file