Well, nobody mentioned this, so you will hear it from the less appropriate person, but Interfaces are also a way to get some pseudo multiple inheritance (which it can be also bad design) for example (and it will probably stink) You might have two classes, Boat and Car, both deriving from Vehicle but now someone brings you a Car that moves in the water (or a boat that moves in the land), problem is you can not derive the new class from both (some languages do allow to do so) so the Interfaces come to the rescue, you could define a class CarBoat from Vehicle that implements IBoat and ICar. Well, that is a crude explanation, someone will give you a better example and explanation.