Detection of and reaction to collisions is a very fundamental element in games. As soon as it has moving components of some sort, it is likely that collisions are of concern. For example, the player character may collide with the scenery of the game. The player may also collide with other characters (enemies, monsters etc.), and of course many games work with projectiles that can hit various other parts on the game screen. But what exactly are collisions in a game, and what are good ways to detect and handle them? This article tries to give answers.