diff --git a/src/ComponentDepot.cs b/src/ComponentDepot.cs
index 4c063ea..d5cb094 100644
--- a/src/ComponentDepot.cs
+++ b/src/ComponentDepot.cs
@@ -221,36 +221,31 @@ namespace MoonTools.ECS
 			TypesWithDisabledSerialization.Add(typeof(TComponent));
 		}
 
-		public ComponentDepotState CreateState()
-		{
-			return new ComponentDepotState();
-		}
-
 		public void Serialize(ComponentDepotState state)
 		{
-			// FIXME: this is creating garbage
-			state.StorageStates.Clear();
 			foreach (var (type, storage) in storages)
 			{
-				// FIXME: we could cache this
 				if (!TypesWithDisabledSerialization.Contains(type))
 				{
-					var storageState = storage.CreateState();
-					storage.Serialize(storageState);
-					state.StorageStates.Add(type, storageState);
+					if (!state.StorageStates.ContainsKey(type))
+					{
+						state.StorageStates.Add(type, storage.CreateState());
+					}
+
+					storage.Serialize(state.StorageStates[type]);
 				}
 			}
 
-			// FIXME: this is creating garbage
-			state.FilterStates.Clear();
 			foreach (var (signature, set) in filterSignatureToEntityIDs)
 			{
 				// FIXME: we could cache this
 				if (!signature.Included.Overlaps(TypesWithDisabledSerialization) && !signature.Excluded.Overlaps(TypesWithDisabledSerialization))
 				{
-					var setState = new IndexableSetState<int>(set.Count);
-					set.Save(setState);
-					state.FilterStates[signature] = setState;
+					if (!state.FilterStates.ContainsKey(signature))
+					{
+						state.FilterStates[signature] = new IndexableSetState<int>(set.Count);
+					}
+					set.Save(state.FilterStates[signature]);
 				}
 			}
 		}
diff --git a/src/EntityStorage.cs b/src/EntityStorage.cs
index 5d48a49..6e598a4 100644
--- a/src/EntityStorage.cs
+++ b/src/EntityStorage.cs
@@ -1,22 +1,75 @@
-namespace MoonTools.ECS
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
 {
 	internal class EntityStorage
 	{
-		public IDStorage idStorage = new IDStorage();
+		private int nextID = 0;
+		private readonly Stack<int> availableIDs = new Stack<int>();
+		private readonly HashSet<int> availableIDHash = new HashSet<int>();
 
 		public Entity Create()
 		{
-			return new Entity(idStorage.NextID());
+			return new Entity(NextID());
 		}
 
 		public bool Exists(in Entity entity)
 		{
-			return idStorage.Taken(entity.ID);
+			return Taken(entity.ID);
 		}
 
 		public void Destroy(in Entity entity)
 		{
-			idStorage.Release(entity.ID);
+			Release(entity.ID);
+		}
+
+		public void Serialize(EntityStorageState state)
+		{
+			state.NextID = nextID;
+			state.availableIDs.Clear();
+			foreach (var id in availableIDs)
+			{
+				state.availableIDs.Add(id);
+			}
+		}
+
+		public void Deserialize(EntityStorageState state)
+		{
+			nextID = state.NextID;
+			availableIDs.Clear();
+			availableIDHash.Clear();
+			foreach (var id in state.availableIDs)
+			{
+				availableIDs.Push(id);
+				availableIDHash.Add(id);
+			}
+		}
+
+		private int NextID()
+		{
+			if (availableIDs.Count > 0)
+			{
+				var id = availableIDs.Pop();
+				availableIDHash.Remove(id);
+				return id;
+			}
+			else
+			{
+				var id = nextID;
+				nextID += 1;
+				return id;
+			}
+		}
+
+		private bool Taken(int id)
+		{
+			return !availableIDHash.Contains(id) && id < nextID;
+		}
+
+		private void Release(int id)
+		{
+			availableIDs.Push(id);
+			availableIDHash.Add(id);
 		}
 	}
 }
diff --git a/src/IDStorage.cs b/src/IDStorage.cs
deleted file mode 100644
index 93e17bf..0000000
--- a/src/IDStorage.cs
+++ /dev/null
@@ -1,39 +0,0 @@
-using System.Collections.Generic;
-
-namespace MoonTools.ECS
-{
-	internal class IDStorage
-	{
-		private int nextID = 0;
-
-		private readonly Stack<int> availableIDs = new Stack<int>();
-		private readonly HashSet<int> availableIDHash = new HashSet<int>();
-
-		public int NextID()
-		{
-			if (availableIDs.Count > 0)
-			{
-				var id = availableIDs.Pop();
-				availableIDHash.Remove(id);
-				return id;
-			}
-			else
-			{
-				var id = nextID;
-				nextID += 1;
-				return id;
-			}
-		}
-
-		public bool Taken(int id)
-		{
-			return !availableIDHash.Contains(id) && id < nextID;
-		}
-
-		public void Release(int id)
-		{
-			availableIDs.Push(id);
-			availableIDHash.Add(id);
-		}
-	}
-}
diff --git a/src/ComponentDepotState.cs b/src/State/ComponentDepotState.cs
similarity index 90%
rename from src/ComponentDepotState.cs
rename to src/State/ComponentDepotState.cs
index b137f24..2370dc6 100644
--- a/src/ComponentDepotState.cs
+++ b/src/State/ComponentDepotState.cs
@@ -3,7 +3,7 @@ using System.Collections.Generic;
 
 namespace MoonTools.ECS
 {
-	public class ComponentDepotState
+	internal class ComponentDepotState
 	{
 		public Dictionary<Type, ComponentStorageState> StorageStates = new Dictionary<Type, ComponentStorageState>();
 		public Dictionary<FilterSignature, IndexableSetState<int>> FilterStates = new Dictionary<FilterSignature, IndexableSetState<int>>();
diff --git a/src/ComponentStorageState.cs b/src/State/ComponentStorageState.cs
similarity index 89%
rename from src/ComponentStorageState.cs
rename to src/State/ComponentStorageState.cs
index dfc3e52..131f83c 100644
--- a/src/ComponentStorageState.cs
+++ b/src/State/ComponentStorageState.cs
@@ -2,8 +2,7 @@ using System.Collections.Generic;
 
 namespace MoonTools.ECS
 {
-	// for saving and loading component storage
-    public class ComponentStorageState
+    internal class ComponentStorageState
     {
 		public int Count;
 		public Dictionary<int, int> EntityIdToStorageIndex;
diff --git a/src/State/EntityStorageState.cs b/src/State/EntityStorageState.cs
new file mode 100644
index 0000000..66363dc
--- /dev/null
+++ b/src/State/EntityStorageState.cs
@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+    internal class EntityStorageState
+    {
+		public int NextID;
+		public List<int> availableIDs = new List<int>();
+	}
+}
diff --git a/src/IndexableSetState.cs b/src/State/IndexableSetState.cs
similarity index 76%
rename from src/IndexableSetState.cs
rename to src/State/IndexableSetState.cs
index 43b7b79..378b9de 100644
--- a/src/IndexableSetState.cs
+++ b/src/State/IndexableSetState.cs
@@ -1,9 +1,8 @@
 using System.Collections.Generic;
-using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
-    public class IndexableSetState<T> where T : unmanaged
+    internal class IndexableSetState<T> where T : unmanaged
     {
 		public int Count;
 		public Dictionary<T, int> Indices;
diff --git a/src/State/WorldState.cs b/src/State/WorldState.cs
new file mode 100644
index 0000000..dd2728a
--- /dev/null
+++ b/src/State/WorldState.cs
@@ -0,0 +1,14 @@
+namespace MoonTools.ECS
+{
+    public class WorldState
+    {
+		internal ComponentDepotState ComponentDepotState;
+		internal EntityStorageState EntityStorageState;
+
+		public WorldState()
+        {
+			ComponentDepotState = new ComponentDepotState();
+			EntityStorageState = new EntityStorageState();
+		}
+	}
+}
diff --git a/src/World.cs b/src/World.cs
index a5dbe81..7ad8c4a 100644
--- a/src/World.cs
+++ b/src/World.cs
@@ -32,16 +32,21 @@
 			ComponentDepot.DisableSerialization<TComponent>();
 		}
 
-		public ComponentDepotState Serialize()
+		public WorldState CreateState()
 		{
-			var state = ComponentDepot.CreateState();
-			ComponentDepot.Serialize(state);
-			return state;
+			return new WorldState();
 		}
 
-		public void Deserialize(ComponentDepotState state)
+		public void Serialize(WorldState state)
 		{
-			ComponentDepot.Deserialize(state);
+			ComponentDepot.Serialize(state.ComponentDepotState);
+			EntityStorage.Serialize(state.EntityStorageState);
+		}
+
+		public void Deserialize(WorldState state)
+		{
+			ComponentDepot.Deserialize(state.ComponentDepotState);
+			EntityStorage.Deserialize(state.EntityStorageState);
 		}
 	}
 }