Collision Response in Entity Components Systems This question seems to be a duplicate of mine, but I don't think it is. I'm trying to build a game using an ECS, but I want this ECS to be as simple as possible, therefore I am eschewing messages. My question is where should the the behaviours go for collision responses. Not physical responses, but logical response (e.g. bullet hurts player, player picks up health, explosion destroys enemies). Is there a big HandleCollisionsSystem, or does each type of collision have it's own system or should the logic be in one or both of the colliding entity's components (but a component is just data, right?) I'm sure there are several approaches, but for the moment I would like to err on the side of simplicity.