The game engine is the program which contains the CAOS interpreter, biochemistry, brain, genetics and user-interface code - essentially all the programming except applets (Creatures and Creatures 2) and CAOS scripts. Different games have used different engines:
- Creatures used its own engine.
- Creatures 2 used an updated version of the Creatures engine.
- Creatures Adventures and Creatures Playground used an early version of the Creatures Evolution Engine.
- Creatures 3 used an updated version of the Creatures Evolution Engine, and Docking Station introduced some minor engine updates (you can run C3 under DS's engine), as well as extending the engine with networking code.
- Amazing Virtual Sea-Monkeys used a further modified version of the Creatures Evolution Engine, without the Docking Station networking code.
- BAMZOOKi uses a completely different engine, and is accompanied by scripts in Lua rather than CAOS.
- Creatures Online uses the Mojito engine.
Also, openc2e is an open-source Creatures engine.