diff --git a/source/Game.h b/source/Game.h
index f38d98d..1c8bef8 100644
--- a/source/Game.h
+++ b/source/Game.h
@@ -19,6 +19,9 @@
 #include "sprite/Player.h"
 #include "util/Collection.h"
 
+class Player;
+class Collection;
+
 /*
  * Use vertex for tiles.
  */
diff --git a/source/TileManager.h b/source/TileManager.h
index d555770..2a7c4cb 100755
--- a/source/TileManager.h
+++ b/source/TileManager.h
@@ -19,6 +19,8 @@
 #include "util/Vector.h"
 #include "abstract/Sprite.h"
 
+class Sprite;
+
 class TileManager : public sf::Drawable {
 // Public constants.
 public:
diff --git a/source/abstract/Sprite.h b/source/abstract/Sprite.h
index 2a9b1b1..b345812 100644
--- a/source/abstract/Sprite.h
+++ b/source/abstract/Sprite.h
@@ -15,6 +15,8 @@
 #include "Physical.h"
 #include "../util/Vector.h"
 
+class Physical;
+
 /**
  * Represents a drawable object.
  *
diff --git a/source/effects/Bullet.h b/source/effects/Bullet.h
index d66bc8c..eb0b858 100755
--- a/source/effects/Bullet.h
+++ b/source/effects/Bullet.h
@@ -10,6 +10,8 @@
 
 #include "../particle/Particle.h"
 
+class Particle;
+
 /**
  * Bullet particle fired by a weapon, may damage actors.
  */
diff --git a/source/items/Weapon.h b/source/items/Weapon.h
index aa1b1c1..76d43ab 100755
--- a/source/items/Weapon.h
+++ b/source/items/Weapon.h
@@ -13,6 +13,9 @@
 #include "../abstract/Physical.h"
 #include "../particle/Emitter.h"
 
+class Physical;
+class Emitter;
+
 /**
  * Loading mechanism:
  * 	- pass enum value and load mapped xml
diff --git a/source/particle/Emitter.h b/source/particle/Emitter.h
index a7cc41b..56cdb06 100755
--- a/source/particle/Emitter.h
+++ b/source/particle/Emitter.h
@@ -12,6 +12,10 @@
 #include "../util/Collection.h"
 #include "Particle.h"
 
+class Physical;
+class Collection;
+class Particle;
+
 class Emitter {
 // Public functions.
 public:
diff --git a/source/particle/Particle.h b/source/particle/Particle.h
index 2f8b4b8..e8ad788 100755
--- a/source/particle/Particle.h
+++ b/source/particle/Particle.h
@@ -10,6 +10,8 @@
 
 #include "../abstract/Sprite.h"
 
+class Sprite;
+
 /**
  * Prototype for a particle.
  */
diff --git a/source/sprite/Body.h b/source/sprite/Body.h
index 2697886..34c006f 100755
--- a/source/sprite/Body.h
+++ b/source/sprite/Body.h
@@ -10,6 +10,8 @@
 
 #include "../abstract/Sprite.h"
 
+class Sprite;
+
 class Body : public Sprite {
 // Public functions.
 public:
diff --git a/source/sprite/Cover.h b/source/sprite/Cover.h
index 35f1eb2..d438980 100755
--- a/source/sprite/Cover.h
+++ b/source/sprite/Cover.h
@@ -10,6 +10,8 @@
 
 #include "../abstract/Sprite.h"
 
+class Sprite;
+
 /**
  * A wall that can be placed anywhere (not limited by tiles) and have any (rectangular) size.
  */
diff --git a/source/sprite/Enemy.h b/source/sprite/Enemy.h
index fd5e53d..964a1b6 100644
--- a/source/sprite/Enemy.h
+++ b/source/sprite/Enemy.h
@@ -13,6 +13,10 @@
 #include "../util/Collection.h"
 #include "../util/Vector.h"
 
+class Actor;
+class Sprite;
+class Collection;
+
 class Enemy : public Sprite, public Actor {
 // Public functions.
 public:
diff --git a/source/sprite/Player.h b/source/sprite/Player.h
index 0a08c6c..b2601ef 100644
--- a/source/sprite/Player.h
+++ b/source/sprite/Player.h
@@ -16,7 +16,9 @@
 #include "../items/Weapon.h"
 #include "../util/Vector.h"
 
+class Actor;
 class Sprite;
+class Weapon;
 
 /**
  * Player object.
diff --git a/source/util/Collection.h b/source/util/Collection.h
index 1ce1fa1..be7d778 100755
--- a/source/util/Collection.h
+++ b/source/util/Collection.h
@@ -15,6 +15,8 @@
 
 #include "../abstract/Sprite.h"
 
+class Sprite;
+
 /**
  * A collection of sprites, which can be put into different layers.
  */
diff --git a/source/util/Loader.h b/source/util/Loader.h
index 753a8a5..16e5914 100755
--- a/source/util/Loader.h
+++ b/source/util/Loader.h
@@ -18,6 +18,9 @@
 
 #include "Singleton.h"
 
+template <class T>
+class Singleton;
+
 /**
  * This class allows to set default resource folders and subfolders, which means that these
  * folders only have to be set once, and not be included in the creation of every single
diff --git a/source/util/ResourceManager.h b/source/util/ResourceManager.h
index b84289a..053de44 100644
--- a/source/util/ResourceManager.h
+++ b/source/util/ResourceManager.h
@@ -14,6 +14,10 @@
 
 #include "Singleton.h"
 
+template <class T>
+class Singleton;
+
+
 /**
  * Loads and manages all resources by providing Singleton access to Thor ResourceManager.
  */