diff --git a/MoonTools.ECS.csproj b/MoonTools.ECS.csproj
index 8867478..e7a1568 100644
--- a/MoonTools.ECS.csproj
+++ b/MoonTools.ECS.csproj
@@ -4,6 +4,7 @@
     <TargetFramework>net6.0</TargetFramework>
     <Nullable>enable</Nullable>
     <Platforms>x64</Platforms>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
 </Project>
diff --git a/src/ComponentDepot.cs b/src/ComponentDepot.cs
index 482214e..a5ada46 100644
--- a/src/ComponentDepot.cs
+++ b/src/ComponentDepot.cs
@@ -11,13 +11,13 @@ namespace MoonTools.ECS
 
 		private Dictionary<Type, HashSet<FilterSignature>> typeToFilterSignatures = new Dictionary<Type, HashSet<FilterSignature>>();
 
-		private Dictionary<int, HashSet<Type>> entityComponentMap = new Dictionary<int, HashSet<Type>>();
-
 #if DEBUG
 		private Dictionary<Type, Filter> singleComponentFilters = new Dictionary<Type, Filter>();
 #endif
 
-		internal void Register<TComponent>() where TComponent : struct
+		private HashSet<Type> TypesWithDisabledSerialization = new HashSet<Type>();
+
+		internal void Register<TComponent>() where TComponent : unmanaged
 		{
 			if (!storages.ContainsKey(typeof(TComponent)))
 			{
@@ -33,19 +33,19 @@ namespace MoonTools.ECS
 			return storages[type];
 		}
 
-		private ComponentStorage<TComponent> Lookup<TComponent>() where TComponent : struct
+		private ComponentStorage<TComponent> Lookup<TComponent>() where TComponent : unmanaged
 		{
 			// TODO: is it possible to optimize this?
 			Register<TComponent>();
 			return (ComponentStorage<TComponent>) storages[typeof(TComponent)];
 		}
 
-		public bool Some<TComponent>() where TComponent : struct
+		public bool Some<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().Any();
 		}
 
-		public bool Has<TComponent>(int entityID) where TComponent : struct
+		public bool Has<TComponent>(int entityID) where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().Has(entityID);
 		}
@@ -55,29 +55,22 @@ namespace MoonTools.ECS
 			return Lookup(type).Has(entityID);
 		}
 
-		public ref readonly TComponent Get<TComponent>(int entityID) where TComponent : struct
+		public ref readonly TComponent Get<TComponent>(int entityID) where TComponent : unmanaged
 		{
 			return ref Lookup<TComponent>().Get(entityID);
 		}
 
-		public ref readonly TComponent Get<TComponent>() where TComponent : struct
+		public ref readonly TComponent Get<TComponent>() where TComponent : unmanaged
 		{
 			return ref Lookup<TComponent>().Get();
 		}
 
-		public void Set<TComponent>(int entityID, in TComponent component) where TComponent : struct
+		public void Set<TComponent>(int entityID, in TComponent component) where TComponent : unmanaged
 		{
-			Lookup<TComponent>().Set(entityID, component);
-
-			if (!entityComponentMap.ContainsKey(entityID))
-			{
-				entityComponentMap.Add(entityID, new HashSet<Type>());
-			}
-
-			var notFound = entityComponentMap[entityID].Add(typeof(TComponent));
+			var existed = Lookup<TComponent>().Set(entityID, component);
 
 			// update filters
-			if (notFound)
+			if (!existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(typeof(TComponent), out var filterSignatures))
 				{
@@ -89,24 +82,22 @@ namespace MoonTools.ECS
 			}
 		}
 
-		public Entity GetSingletonEntity<TComponent>() where TComponent : struct
+		public Entity GetSingletonEntity<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().FirstEntity();
 		}
 
-		public ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : struct
+		public ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().AllComponents();
 		}
 
 		private void Remove(Type type, int entityID)
 		{
-			Lookup(type).Remove(entityID);
-
-			var found = entityComponentMap[entityID].Remove(type);
+			var existed = Lookup(type).Remove(entityID);
 
 			// update filters
-			if (found)
+			if (existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(type, out var filterSignatures))
 				{
@@ -118,14 +109,12 @@ namespace MoonTools.ECS
 			}
 		}
 
-		public void Remove<TComponent>(int entityID) where TComponent : struct
+		public void Remove<TComponent>(int entityID) where TComponent : unmanaged
 		{
-			Lookup<TComponent>().Remove(entityID);
-
-			var found = entityComponentMap[entityID].Remove(typeof(TComponent));
+			var existed = Lookup<TComponent>().Remove(entityID);
 
 			// update filters
-			if (found)
+			if (existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(typeof(TComponent), out var filterSignatures))
 				{
@@ -137,16 +126,12 @@ namespace MoonTools.ECS
 			}
 		}
 
+		// TODO: is there some way to optimize this without complicating serialization?
 		public void OnEntityDestroy(int entityID)
 		{
-			if (entityComponentMap.ContainsKey(entityID))
+			foreach (var type in storages.Keys)
 			{
-				foreach (var type in entityComponentMap[entityID])
-				{
-					Remove(type, entityID);
-				}
-
-				entityComponentMap.Remove(entityID);
+				Remove(type, entityID);
 			}
 		}
 
@@ -231,6 +216,53 @@ namespace MoonTools.ECS
 			filterSignatureToEntityIDs[filterSignature].Add(entityID);
 		}
 
+		public void DisableSerialization<TComponent>() where TComponent : unmanaged
+		{
+			TypesWithDisabledSerialization.Add(typeof(TComponent));
+		}
+
+		public void Save(ComponentDepotState state)
+		{
+			foreach (var (type, storage) in storages)
+			{
+				if (!TypesWithDisabledSerialization.Contains(type))
+				{
+					if (!state.StorageStates.ContainsKey(type))
+					{
+						state.StorageStates.Add(type, storage.CreateState());
+					}
+
+					storage.Save(state.StorageStates[type]);
+				}
+			}
+
+			foreach (var (signature, set) in filterSignatureToEntityIDs)
+			{
+				// FIXME: we could cache this
+				if (!signature.Included.Overlaps(TypesWithDisabledSerialization) && !signature.Excluded.Overlaps(TypesWithDisabledSerialization))
+				{
+					if (!state.FilterStates.ContainsKey(signature))
+					{
+						state.FilterStates[signature] = new IndexableSetState<int>(set.Count);
+					}
+					set.Save(state.FilterStates[signature]);
+				}
+			}
+		}
+
+		public void Load(ComponentDepotState state)
+		{
+			foreach (var (type, storageState) in state.StorageStates)
+			{
+				storages[type].Load(storageState);
+			}
+
+			foreach (var (signature, setState) in state.FilterStates)
+			{
+				filterSignatureToEntityIDs[signature].Load(setState);
+			}
+		}
+
 #if DEBUG
 		public IEnumerable<object> Debug_GetAllComponents(int entityID)
 		{
diff --git a/src/ComponentStorage.cs b/src/ComponentStorage.cs
index 15a5db2..c7d256b 100644
--- a/src/ComponentStorage.cs
+++ b/src/ComponentStorage.cs
@@ -1,17 +1,20 @@
 using System;
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
 	internal abstract class ComponentStorage
 	{
 		public abstract bool Has(int entityID);
-		public abstract void Remove(int entityID);
+		public abstract bool Remove(int entityID);
 		public abstract object Debug_Get(int entityID);
+		public abstract ComponentStorageState CreateState();
+		public abstract void Save(ComponentStorageState state);
+		public abstract void Load(ComponentStorageState state);
 	}
 
-	// FIXME: we can probably get rid of this weird entity storage system by using filters
-	internal class ComponentStorage<TComponent> : ComponentStorage where TComponent : struct
+	internal class ComponentStorage<TComponent> : ComponentStorage where TComponent : unmanaged
 	{
 		private int nextID;
 		private readonly Dictionary<int, int> entityIDToStorageIndex = new Dictionary<int, int>(16);
@@ -49,8 +52,11 @@ namespace MoonTools.ECS
 			return ref components[0];
 		}
 
-		public void Set(int entityID, in TComponent component)
+		// Returns true if the entity already had this component.
+		public bool Set(int entityID, in TComponent component)
 		{
+			bool result = true;
+
 			if (!entityIDToStorageIndex.ContainsKey(entityID))
 			{
 				var index = nextID;
@@ -64,12 +70,17 @@ namespace MoonTools.ECS
 
 				entityIDToStorageIndex[entityID] = index;
 				entityIDs[index] = entityID;
+
+				result = false;
 			}
 
 			components[entityIDToStorageIndex[entityID]] = component;
+
+			return result;
 		}
 
-		public override void Remove(int entityID)
+		// Returns true if the entity had this component.
+		public override bool Remove(int entityID)
 		{
 			if (entityIDToStorageIndex.ContainsKey(entityID))
 			{
@@ -88,7 +99,11 @@ namespace MoonTools.ECS
 				}
 
 				nextID -= 1;
+
+				return true;
 			}
+
+			return false;
 		}
 
 		public void Clear()
@@ -106,5 +121,44 @@ namespace MoonTools.ECS
 		{
 			return new Entity(entityIDs[0]);
 		}
+
+		public override ComponentStorageState CreateState()
+		{
+			return ComponentStorageState.Create<TComponent>(nextID);
+		}
+
+		public override void Save(ComponentStorageState state)
+		{
+			ReadOnlySpan<byte> entityIDBytes = MemoryMarshal.Cast<int, byte>(new ReadOnlySpan<int>(entityIDs, 0, nextID));
+
+			if (entityIDBytes.Length > state.EntityIDs.Length)
+			{
+				Array.Resize(ref state.EntityIDs, entityIDBytes.Length);
+			}
+			entityIDBytes.CopyTo(state.EntityIDs);
+
+			ReadOnlySpan<byte> componentBytes = MemoryMarshal.Cast<TComponent, byte>(AllComponents());
+			if (componentBytes.Length > state.Components.Length)
+			{
+				Array.Resize(ref state.Components, componentBytes.Length);
+			}
+			componentBytes.CopyTo(state.Components);
+
+			state.Count = nextID;
+		}
+
+		public override void Load(ComponentStorageState state)
+		{
+			state.EntityIDs.CopyTo(MemoryMarshal.Cast<int, byte>(entityIDs));
+			state.Components.CopyTo(MemoryMarshal.Cast<TComponent, byte>(components));
+
+			entityIDToStorageIndex.Clear();
+			for (var i = 0; i < state.Count; i += 1)
+			{
+				entityIDToStorageIndex[entityIDs[i]] = i;
+			}
+
+			nextID = state.Count;
+		}
 	}
 }
diff --git a/src/EntityComponentReader.cs b/src/EntityComponentReader.cs
index 5449d95..b39de4f 100644
--- a/src/EntityComponentReader.cs
+++ b/src/EntityComponentReader.cs
@@ -5,52 +5,43 @@ namespace MoonTools.ECS
 {
 	public abstract class EntityComponentReader
 	{
-		internal EntityStorage EntityStorage;
-		internal ComponentDepot ComponentDepot;
-		internal RelationDepot RelationDepot;
+		protected readonly World World;
+		internal EntityStorage EntityStorage => World.EntityStorage;
+		internal ComponentDepot ComponentDepot => World.ComponentDepot;
+		internal RelationDepot RelationDepot => World.RelationDepot;
 		protected FilterBuilder FilterBuilder => new FilterBuilder(ComponentDepot);
 
-		internal void RegisterEntityStorage(EntityStorage entityStorage)
+		public EntityComponentReader(World world)
 		{
-			EntityStorage = entityStorage;
+			World = world;
 		}
 
-		internal void RegisterComponentDepot(ComponentDepot componentDepot)
-		{
-			ComponentDepot = componentDepot;
-		}
-
-		internal void RegisterRelationDepot(RelationDepot relationDepot)
-		{
-			RelationDepot = relationDepot;
-		}
-
-		protected ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : struct
+		protected ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : unmanaged
 		{
 			return ComponentDepot.ReadComponents<TComponent>();
 		}
 
-		protected bool Has<TComponent>(in Entity entity) where TComponent : struct
+		protected bool Has<TComponent>(in Entity entity) where TComponent : unmanaged
 		{
 			return ComponentDepot.Has<TComponent>(entity.ID);
 		}
 
-		protected bool Some<TComponent>() where TComponent : struct
+		protected bool Some<TComponent>() where TComponent : unmanaged
 		{
 			return ComponentDepot.Some<TComponent>();
 		}
 
-		protected ref readonly TComponent Get<TComponent>(in Entity entity) where TComponent : struct
+		protected ref readonly TComponent Get<TComponent>(in Entity entity) where TComponent : unmanaged
 		{
 			return ref ComponentDepot.Get<TComponent>(entity.ID);
 		}
 
-		protected ref readonly TComponent GetSingleton<TComponent>() where TComponent : struct
+		protected ref readonly TComponent GetSingleton<TComponent>() where TComponent : unmanaged
 		{
 			return ref ComponentDepot.Get<TComponent>();
 		}
 
-		protected Entity GetSingletonEntity<TComponent>() where TComponent : struct
+		protected Entity GetSingletonEntity<TComponent>() where TComponent : unmanaged
 		{
 			return ComponentDepot.GetSingletonEntity<TComponent>();
 		}
@@ -60,22 +51,22 @@ namespace MoonTools.ECS
 			return EntityStorage.Exists(entity);
 		}
 
-		protected IEnumerable<(Entity, Entity, TRelationKind)> Relations<TRelationKind>() where TRelationKind : struct
+		protected IEnumerable<(Entity, Entity, TRelationKind)> Relations<TRelationKind>() where TRelationKind : unmanaged
 		{
 			return RelationDepot.Relations<TRelationKind>();
 		}
 
-		protected bool Related<TRelationKind>(in Entity a, in Entity b) where TRelationKind : struct
+		protected bool Related<TRelationKind>(in Entity a, in Entity b) where TRelationKind : unmanaged
 		{
 			return RelationDepot.Related<TRelationKind>(a.ID, b.ID);
 		}
 
-		protected IEnumerable<(Entity, TRelationKind)> RelatedToA<TRelationKind>(in Entity entity) where TRelationKind : struct
+		protected IEnumerable<(Entity, TRelationKind)> RelatedToA<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.RelatedToA<TRelationKind>(entity.ID);
 		}
 
-		protected IEnumerable<(Entity, TRelationKind)> RelatedToB<TRelationKind>(in Entity entity) where TRelationKind : struct
+		protected IEnumerable<(Entity, TRelationKind)> RelatedToB<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.RelatedToB<TRelationKind>(entity.ID);
 		}
diff --git a/src/EntityStorage.cs b/src/EntityStorage.cs
index 5d48a49..2379b5d 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 Save(EntityStorageState state)
+		{
+			state.NextID = nextID;
+			state.availableIDs.Clear();
+			foreach (var id in availableIDs)
+			{
+				state.availableIDs.Add(id);
+			}
+		}
+
+		public void Load(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/FilterBuilder.cs b/src/FilterBuilder.cs
index 4ba782d..be75edc 100644
--- a/src/FilterBuilder.cs
+++ b/src/FilterBuilder.cs
@@ -23,14 +23,14 @@ namespace MoonTools.ECS
 			Excluded = excluded;
 		}
 
-		public FilterBuilder Include<TComponent>() where TComponent : struct
+		public FilterBuilder Include<TComponent>() where TComponent : unmanaged
 		{
 			ComponentDepot.Register<TComponent>();
 			Included.Add(typeof(TComponent));
 			return new FilterBuilder(ComponentDepot, Included, Excluded);
 		}
 
-		public FilterBuilder Exclude<TComponent>() where TComponent : struct
+		public FilterBuilder Exclude<TComponent>() where TComponent : unmanaged
 		{
 			ComponentDepot.Register<TComponent>();
 			Excluded.Add(typeof(TComponent));
diff --git a/src/FilterSignature.cs b/src/FilterSignature.cs
index 4857119..7508b36 100644
--- a/src/FilterSignature.cs
+++ b/src/FilterSignature.cs
@@ -7,8 +7,8 @@ namespace MoonTools.ECS
 	{
 		private const int HASH_FACTOR = 97;
 
-		public HashSet<Type> Included;
-		public HashSet<Type> Excluded;
+		public readonly HashSet<Type> Included;
+		public readonly HashSet<Type> Excluded;
 
 		public FilterSignature(HashSet<Type> included, HashSet<Type> excluded)
 		{
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/IndexableSet.cs b/src/IndexableSet.cs
index cf8f56c..275044a 100644
--- a/src/IndexableSet.cs
+++ b/src/IndexableSet.cs
@@ -1,10 +1,11 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
-	internal class IndexableSet<T> : IEnumerable<T> where T : notnull
+	internal class IndexableSet<T> : IEnumerable<T> where T : unmanaged
 	{
 		private Dictionary<T, int> indices;
 		private T[] array;
@@ -78,5 +79,32 @@ namespace MoonTools.ECS
 				yield return array[i];
 			}
 		}
+
+		public void Save(IndexableSetState<T> state)
+		{
+			ReadOnlySpan<byte> arrayBytes = MemoryMarshal.Cast<T, byte>(array);
+
+			if (arrayBytes.Length > state.Array.Length)
+			{
+				Array.Resize(ref state.Array, arrayBytes.Length);
+			}
+
+			arrayBytes.CopyTo(state.Array);
+
+			state.Count = Count;
+		}
+
+		public void Load(IndexableSetState<T> state)
+		{
+			state.Array.CopyTo(MemoryMarshal.Cast<T, byte>(array));
+
+			indices.Clear();
+			for (var i = 0; i < state.Count; i += 1)
+			{
+				indices[array[i]] = i;
+			}
+
+			Count = state.Count;
+		}
 	}
 }
diff --git a/src/RelationDepot.cs b/src/RelationDepot.cs
index 82b7114..c3a65b3 100644
--- a/src/RelationDepot.cs
+++ b/src/RelationDepot.cs
@@ -7,7 +7,7 @@ namespace MoonTools.ECS
 	{
 		private Dictionary<Type, RelationStorage> storages = new Dictionary<Type, RelationStorage>();
 
-		private void Register<TRelationKind>() where TRelationKind : struct
+		private void Register<TRelationKind>() where TRelationKind : unmanaged
 		{
 			if (!storages.ContainsKey(typeof(TRelationKind)))
 			{
@@ -15,18 +15,18 @@ namespace MoonTools.ECS
 			}
 		}
 
-		private RelationStorage<TRelationKind> Lookup<TRelationKind>() where TRelationKind : struct
+		private RelationStorage<TRelationKind> Lookup<TRelationKind>() where TRelationKind : unmanaged
 		{
 			Register<TRelationKind>();
 			return (RelationStorage<TRelationKind>) storages[typeof(TRelationKind)];
 		}
 
-		public void Add<TRelationKind>(Relation relation, TRelationKind relationData) where TRelationKind : struct
+		public void Set<TRelationKind>(Relation relation, TRelationKind relationData) where TRelationKind : unmanaged
 		{
-			Lookup<TRelationKind>().Add(relation, relationData);
+			Lookup<TRelationKind>().Set(relation, relationData);
 		}
 
-		public void Remove<TRelationKind>(Relation relation) where TRelationKind : struct
+		public void Remove<TRelationKind>(Relation relation) where TRelationKind : unmanaged
 		{
 			Lookup<TRelationKind>().Remove(relation);
 		}
@@ -40,24 +40,45 @@ namespace MoonTools.ECS
 			}
 		}
 
-		public IEnumerable<(Entity, Entity, TRelationKind)> Relations<TRelationKind>() where TRelationKind : struct
+		public IEnumerable<(Entity, Entity, TRelationKind)> Relations<TRelationKind>() where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().All();
 		}
 
-		public bool Related<TRelationKind>(int idA, int idB) where TRelationKind : struct
+		public bool Related<TRelationKind>(int idA, int idB) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().Has(new Relation(idA, idB));
 		}
 
-		public IEnumerable<(Entity, TRelationKind)> RelatedToA<TRelationKind>(int entityID) where TRelationKind : struct
+		public IEnumerable<(Entity, TRelationKind)> RelatedToA<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().RelatedToA(entityID);
 		}
 
-		public IEnumerable<(Entity, TRelationKind)> RelatedToB<TRelationKind>(int entityID) where TRelationKind : struct
+		public IEnumerable<(Entity, TRelationKind)> RelatedToB<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().RelatedToB(entityID);
 		}
+
+		public void Save(RelationDepotState state)
+		{
+			foreach (var (type, storage) in storages)
+			{
+				if (!state.StorageStates.ContainsKey(type))
+				{
+					state.StorageStates.Add(type, storage.CreateState());
+				}
+
+				storage.Save(state.StorageStates[type]);
+			}
+		}
+
+		public void Load(RelationDepotState state)
+		{
+			foreach (var (type, storageState) in state.StorageStates)
+			{
+				storages[type].Load(storageState);
+			}
+		}
 	}
 }
diff --git a/src/RelationStorage.cs b/src/RelationStorage.cs
index 6f82074..9deb5a6 100644
--- a/src/RelationStorage.cs
+++ b/src/RelationStorage.cs
@@ -1,32 +1,46 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
 	internal abstract class RelationStorage
 	{
+		public abstract RelationStorageState CreateState();
+		public abstract void Save(RelationStorageState state);
+		public abstract void Load(RelationStorageState state);
 		public abstract void OnEntityDestroy(int entityID);
 	}
 
 	// Relation is the two entities, A related to B.
 	// TRelation is the data attached to the relation.
-	internal class RelationStorage<TRelation> : RelationStorage where TRelation : struct
+	internal class RelationStorage<TRelation> : RelationStorage where TRelation : unmanaged
 	{
-		private Dictionary<Relation, TRelation> relations = new Dictionary<Relation, TRelation>(16);
+		private int count = 0;
+		private Dictionary<Relation, int> indices = new Dictionary<Relation, int>(16);
+		private Relation[] relations = new Relation[16];
+		private TRelation[] relationDatas = new TRelation[16];
 		private Dictionary<int, HashSet<int>> entitiesRelatedToA = new Dictionary<int, HashSet<int>>(16);
 		private Dictionary<int, HashSet<int>> entitiesRelatedToB = new Dictionary<int, HashSet<int>>(16);
 		private Stack<HashSet<int>> listPool = new Stack<HashSet<int>>();
 
 		public IEnumerable<(Entity, Entity, TRelation)> All()
 		{
-			foreach (var relationData in relations)
+			for (var i = 0; i < count; i += 1)
 			{
-				yield return (relationData.Key.A, relationData.Key.B, relationData.Value);
+				var relation = relations[i];
+				yield return (relation.A, relation.B, relationDatas[i]);
 			}
 		}
 
-		public void Add(Relation relation, TRelation relationData)
+		public void Set(Relation relation, TRelation relationData)
 		{
-			if (relations.ContainsKey(relation)) { return; }
+			if (indices.ContainsKey(relation))
+			{
+				var index = indices[relation];
+				relationDatas[index] = relationData;
+				return;
+			}
 
 			var idA = relation.A.ID;
 			var idB = relation.B.ID;
@@ -43,12 +57,20 @@ namespace MoonTools.ECS
 			}
 			entitiesRelatedToB[idB].Add(idA);
 
-			relations.Add(relation, relationData);
+			if (count >= relationDatas.Length)
+			{
+				Array.Resize(ref relationDatas, relationDatas.Length * 2);
+			}
+
+			relations[count] = relation;
+			relationDatas[count] = relationData;
+			indices.Add(relation, count);
+			count += 1;
 		}
 
 		public bool Has(Relation relation)
 		{
-			return relations.ContainsKey(relation);
+			return indices.ContainsKey(relation);
 		}
 
 		// FIXME: is there a more descriptive name for these?
@@ -59,7 +81,7 @@ namespace MoonTools.ECS
 				foreach (var id in entitiesRelatedToA[entityID])
 				{
 					var relation = new Relation(entityID, id);
-					yield return (relation.B, relations[relation]);
+					yield return (relation.B, relationDatas[indices[relation]]);
 				}
 			}
 		}
@@ -71,7 +93,7 @@ namespace MoonTools.ECS
 				foreach (var id in entitiesRelatedToB[entityID])
 				{
 					var relation = new Relation(id, entityID);
-					yield return (relation.A, relations[relation]);
+					yield return (relation.A, relationDatas[indices[relation]]);
 				}
 			}
 		}
@@ -88,7 +110,26 @@ namespace MoonTools.ECS
 				entitiesRelatedToB[relation.B.ID].Remove(relation.A.ID);
 			}
 
-			return relations.Remove(relation);
+			if (indices.ContainsKey(relation))
+			{
+				var index = indices[relation];
+				var lastElementIndex = count - 1;
+
+				// move an element into the hole
+				if (index != lastElementIndex)
+				{
+					var lastRelation = relations[lastElementIndex];
+					indices[lastRelation] = index;
+					relationDatas[index] = relationDatas[lastElementIndex];
+					relations[index] = lastRelation;
+				}
+
+				count -= 1;
+				indices.Remove(relation);
+				return true;
+			}
+
+			return false;
 		}
 
 		public override void OnEntityDestroy(int entityID)
@@ -131,5 +172,60 @@ namespace MoonTools.ECS
 			hashSet.Clear();
 			listPool.Push(hashSet);
 		}
+
+		public override RelationStorageState CreateState()
+		{
+			return RelationStorageState.Create<TRelation>(count);
+		}
+
+		public override void Save(RelationStorageState state)
+		{
+			ReadOnlySpan<byte> relationBytes = MemoryMarshal.Cast<Relation, byte>(relations);
+
+			if (relationBytes.Length > state.Relations.Length)
+			{
+				Array.Resize(ref state.Relations, relationBytes.Length);
+			}
+			relationBytes.CopyTo(state.Relations);
+
+			ReadOnlySpan<byte> relationDataBytes = MemoryMarshal.Cast<TRelation, byte>(relationDatas);
+
+			if (relationDataBytes.Length > state.RelationDatas.Length)
+			{
+				Array.Resize(ref state.RelationDatas, relationDataBytes.Length);
+			}
+			relationDataBytes.CopyTo(state.RelationDatas);
+
+			state.Count = count;
+		}
+
+		public override void Load(RelationStorageState state)
+		{
+			state.Relations.CopyTo(MemoryMarshal.Cast<Relation, byte>(relations));
+			state.RelationDatas.CopyTo(MemoryMarshal.Cast<TRelation, byte>(relationDatas));
+
+			indices.Clear();
+			entitiesRelatedToA.Clear();
+			entitiesRelatedToB.Clear();
+			for (var i = 0; i < state.Count; i += 1)
+			{
+				var relation = relations[i];
+				indices[relation] = i;
+
+				if (!entitiesRelatedToA.ContainsKey(relation.A.ID))
+				{
+					entitiesRelatedToA[relation.A.ID] = AcquireHashSetFromPool();
+				}
+				entitiesRelatedToA[relation.A.ID].Add(relation.B.ID);
+
+				if (!entitiesRelatedToB.ContainsKey(relation.B.ID))
+				{
+					entitiesRelatedToB[relation.B.ID] = AcquireHashSetFromPool();
+				}
+				entitiesRelatedToB[relation.B.ID].Add(relation.A.ID);
+			}
+
+			count = state.Count;
+		}
 	}
 }
diff --git a/src/Renderer.cs b/src/Renderer.cs
index 07531f3..f6118c2 100644
--- a/src/Renderer.cs
+++ b/src/Renderer.cs
@@ -2,9 +2,6 @@
 {
 	public abstract class Renderer : EntityComponentReader
 	{
-		public Renderer(World world)
-		{
-			world.AddRenderer(this);
-		}
+		public Renderer(World world) : base(world) { }
 	}
 }
diff --git a/src/State/ComponentDepotState.cs b/src/State/ComponentDepotState.cs
new file mode 100644
index 0000000..2370dc6
--- /dev/null
+++ b/src/State/ComponentDepotState.cs
@@ -0,0 +1,11 @@
+using System;
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+	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/State/ComponentStorageState.cs b/src/State/ComponentStorageState.cs
new file mode 100644
index 0000000..ad4872d
--- /dev/null
+++ b/src/State/ComponentStorageState.cs
@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+    internal class ComponentStorageState
+    {
+		public int Count;
+		public byte[] EntityIDs;
+		public byte[] Components;
+
+        public unsafe static ComponentStorageState Create<TComponent>(int count) where TComponent : unmanaged
+        {
+			return new ComponentStorageState(
+                count,
+                count * sizeof(int),
+                count * sizeof(TComponent)
+            );
+		}
+
+        private ComponentStorageState(int count, int entityIDSize, int componentSize)
+        {
+			Count = count;
+			EntityIDs = new byte[entityIDSize];
+			Components = new byte[componentSize];
+		}
+	}
+}
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/State/IndexableSetState.cs b/src/State/IndexableSetState.cs
new file mode 100644
index 0000000..4367252
--- /dev/null
+++ b/src/State/IndexableSetState.cs
@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+    internal class IndexableSetState<T> where T : unmanaged
+    {
+		public int Count;
+		public byte[] Array;
+
+        public unsafe IndexableSetState(int count)
+        {
+			Count = count;
+			Array = new byte[sizeof(T) * count];
+		}
+	}
+}
diff --git a/src/State/RelationDepotState.cs b/src/State/RelationDepotState.cs
new file mode 100644
index 0000000..58e1315
--- /dev/null
+++ b/src/State/RelationDepotState.cs
@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+    internal class RelationDepotState
+    {
+		public Dictionary<Type, RelationStorageState> StorageStates = new Dictionary<Type, RelationStorageState>();
+	}
+}
diff --git a/src/State/RelationStorageState.cs b/src/State/RelationStorageState.cs
new file mode 100644
index 0000000..d210a5d
--- /dev/null
+++ b/src/State/RelationStorageState.cs
@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+
+namespace MoonTools.ECS
+{
+	internal class RelationStorageState
+	{
+		public int Count;
+		public byte[] Relations;
+		public byte[] RelationDatas;
+
+		public unsafe static RelationStorageState Create<TRelation>(int count) where TRelation : unmanaged
+		{
+			return new RelationStorageState(
+				count,
+				count * sizeof(Relation),
+				count * sizeof(TRelation)
+			);
+		}
+
+		private RelationStorageState(int count, int relationSize, int relationDataSize)
+		{
+			Count = count;
+			Relations = new byte[relationSize];
+			RelationDatas = new byte[relationDataSize];
+		}
+	}
+}
diff --git a/src/State/WorldState.cs b/src/State/WorldState.cs
new file mode 100644
index 0000000..d422745
--- /dev/null
+++ b/src/State/WorldState.cs
@@ -0,0 +1,16 @@
+namespace MoonTools.ECS
+{
+    public class WorldState
+    {
+		internal readonly ComponentDepotState ComponentDepotState;
+		internal readonly EntityStorageState EntityStorageState;
+		internal readonly RelationDepotState RelationDepotState;
+
+		public WorldState()
+        {
+			ComponentDepotState = new ComponentDepotState();
+			EntityStorageState = new EntityStorageState();
+			RelationDepotState = new RelationDepotState();
+		}
+	}
+}
diff --git a/src/System.cs b/src/System.cs
index 66eee93..a2749a8 100644
--- a/src/System.cs
+++ b/src/System.cs
@@ -5,17 +5,9 @@ namespace MoonTools.ECS
 {
 	public abstract class System : EntityComponentReader
 	{
-		internal MessageDepot MessageDepot;
+		internal MessageDepot MessageDepot => World.MessageDepot;
 
-		internal void RegisterMessageDepot(MessageDepot messageDepot)
-		{
-			MessageDepot = messageDepot;
-		}
-
-		public System(World world)
-		{
-			world.AddSystem(this);
-		}
+		public System(World world) : base(world) { }
 
 		public abstract void Update(TimeSpan delta);
 
@@ -24,7 +16,7 @@ namespace MoonTools.ECS
 			return EntityStorage.Create();
 		}
 
-		protected void Set<TComponent>(in Entity entity, in TComponent component) where TComponent : struct
+		protected void Set<TComponent>(in Entity entity, in TComponent component) where TComponent : unmanaged
 		{
 #if DEBUG
 			// check for use after destroy
@@ -36,52 +28,52 @@ namespace MoonTools.ECS
 			ComponentDepot.Set<TComponent>(entity.ID, component);
 		}
 
-		protected void Remove<TComponent>(in Entity entity) where TComponent : struct
+		protected void Remove<TComponent>(in Entity entity) where TComponent : unmanaged
 		{
 			ComponentDepot.Remove<TComponent>(entity.ID);
 		}
 
-		protected ReadOnlySpan<TMessage> ReadMessages<TMessage>() where TMessage : struct
+		protected ReadOnlySpan<TMessage> ReadMessages<TMessage>() where TMessage : unmanaged
 		{
 			return MessageDepot.All<TMessage>();
 		}
 
-		protected TMessage ReadMessage<TMessage>() where TMessage : struct
+		protected TMessage ReadMessage<TMessage>() where TMessage : unmanaged
 		{
 			return MessageDepot.First<TMessage>();
 		}
 
-		protected bool SomeMessage<TMessage>() where TMessage : struct
+		protected bool SomeMessage<TMessage>() where TMessage : unmanaged
 		{
 			return MessageDepot.Some<TMessage>();
 		}
 
-		protected IEnumerable<TMessage> ReadMessagesWithEntity<TMessage>(in Entity entity) where TMessage : struct, IHasEntity
+		protected IEnumerable<TMessage> ReadMessagesWithEntity<TMessage>(in Entity entity) where TMessage : unmanaged, IHasEntity
 		{
 			return MessageDepot.WithEntity<TMessage>(entity.ID);
 		}
 
-		protected ref readonly TMessage ReadMessageWithEntity<TMessage>(in Entity entity) where TMessage : struct, IHasEntity
+		protected ref readonly TMessage ReadMessageWithEntity<TMessage>(in Entity entity) where TMessage : unmanaged, IHasEntity
 		{
 			return ref MessageDepot.FirstWithEntity<TMessage>(entity.ID);
 		}
 
-		protected bool SomeMessageWithEntity<TMessage>(in Entity entity) where TMessage : struct, IHasEntity
+		protected bool SomeMessageWithEntity<TMessage>(in Entity entity) where TMessage : unmanaged, IHasEntity
 		{
 			return MessageDepot.SomeWithEntity<TMessage>(entity.ID);
 		}
 
-		protected void Send<TMessage>(in TMessage message) where TMessage : struct
+		protected void Send<TMessage>(in TMessage message) where TMessage : unmanaged
 		{
 			MessageDepot.Add(message);
 		}
 
-		protected void Relate<TRelationKind>(in Entity entityA, in Entity entityB, TRelationKind relationData) where TRelationKind : struct
+		protected void Relate<TRelationKind>(in Entity entityA, in Entity entityB, TRelationKind relationData) where TRelationKind : unmanaged
 		{
-			RelationDepot.Add<TRelationKind>(new Relation(entityA, entityB), relationData);
+			RelationDepot.Set<TRelationKind>(new Relation(entityA, entityB), relationData);
 		}
 
-		protected void Unrelate<TRelationKind>(in Entity entityA, in Entity entityB) where TRelationKind : struct
+		protected void Unrelate<TRelationKind>(in Entity entityA, in Entity entityB) where TRelationKind : unmanaged
 		{
 			RelationDepot.Remove<TRelationKind>(new Relation(entityA, entityB));
 		}
diff --git a/src/World.cs b/src/World.cs
index 8e9e97d..939a9bd 100644
--- a/src/World.cs
+++ b/src/World.cs
@@ -2,37 +2,22 @@
 {
 	public class World
 	{
-		private readonly EntityStorage EntityStorage = new EntityStorage();
-		private readonly ComponentDepot ComponentDepot = new ComponentDepot();
-		private readonly MessageDepot MessageDepot = new MessageDepot();
-		private readonly RelationDepot RelationDepot = new RelationDepot();
-
-		internal void AddSystem(System system)
-		{
-			system.RegisterEntityStorage(EntityStorage);
-			system.RegisterComponentDepot(ComponentDepot);
-			system.RegisterMessageDepot(MessageDepot);
-			system.RegisterRelationDepot(RelationDepot);
-		}
-
-		internal void AddRenderer(Renderer renderer)
-		{
-			renderer.RegisterEntityStorage(EntityStorage);
-			renderer.RegisterComponentDepot(ComponentDepot);
-			renderer.RegisterRelationDepot(RelationDepot);
-		}
+		internal readonly EntityStorage EntityStorage = new EntityStorage();
+		internal readonly ComponentDepot ComponentDepot = new ComponentDepot();
+		internal readonly MessageDepot MessageDepot = new MessageDepot();
+		internal readonly RelationDepot RelationDepot = new RelationDepot();
 
 		public Entity CreateEntity()
 		{
 			return EntityStorage.Create();
 		}
 
-		public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : struct
+		public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : unmanaged
 		{
 			ComponentDepot.Set(entity.ID, component);
 		}
 
-		public void Send<TMessage>(in TMessage message) where TMessage : struct
+		public void Send<TMessage>(in TMessage message) where TMessage : unmanaged
 		{
 			MessageDepot.Add(message);
 		}
@@ -41,5 +26,29 @@
 		{
 			MessageDepot.Clear();
 		}
+
+		public void DisableSerialization<TComponent>() where TComponent : unmanaged
+		{
+			ComponentDepot.DisableSerialization<TComponent>();
+		}
+
+		public WorldState CreateState()
+		{
+			return new WorldState();
+		}
+
+		public void Save(WorldState state)
+		{
+			ComponentDepot.Save(state.ComponentDepotState);
+			EntityStorage.Save(state.EntityStorageState);
+			RelationDepot.Save(state.RelationDepotState);
+		}
+
+		public void Load(WorldState state)
+		{
+			ComponentDepot.Load(state.ComponentDepotState);
+			EntityStorage.Load(state.EntityStorageState);
+			RelationDepot.Load(state.RelationDepotState);
+		}
 	}
 }