Alien Swarm is a space-themed arcade game for the Atari 400/800. The player controls a small vessel on the bottom of the screen which can move horizontally and fire vertically. On the bottom of the screen are a limited number of power packs. A player must grab a power pack to have the energy to fire, and every shot will reduce their energy supply. A red snake-like figure will swarm about, and green bugs will break loose from this, and attempt to steal you limited power packs. If a monster grabs one of these, they can be shot, but the player must be careful not to be hit my the falling pack. If the player shoots the red snake, a bouncing bomb will drop at the player and threaten them. When a player has lost all power packs, it's game over.