Aliens: Colonial Marines is a first-person shooter based on the Aliens franchise and a sequel to the 1986 film Aliens, the second film in the series. The events take place directly after those of the film. The player takes control of the United States Colonial Marine Christopher Winter who is part of a search and rescue team that has to investigate the space ship U.S.S. Sulaco and what remains of Hadley’s Hope to find out what has happened to Ellen Ripley, Dwayne Hicks and the rest of the team that was sent to Acheron (LV-426), a fictional location as one of the three moons of Calpamos, located in the Zeta II Reticuli system. The team arrives on the space ship U.S.S. Sephora seventeen weeks after the distress call by Hicks was sent out.
The single-player campaign consists of eleven missions. Gameplay is centered around shooting opponents and sometimes items in the environment require interaction to open doors and such to progress. Some of the included weapons are a pulse rifle, shotgun, pistol, RPG, flamethrower and smartgun. Sentry turrets can be set up to defend specific areas. The turrets, along with the flamethrower and the smartgun, are not permanently available but are reserved for specific sections. There is no limitation on the amount of weapons that can be carried at a single moment. There is a motion tracker to scan movement in areas 180 degrees in front of the character. Ammo and armour upgrades can be picked up along the way. A flashlight is used to illuminate dark environments and a welder is available to cut open doors. In additional there is an leveling/experience system unified for both single-player and multiplayer where the earned points can be used to unlock new attachments and aesthetic enhancements for weapons and the main character. Most of the missions are not played alone. There is a squad of AI-controlled team mates who assist. They act independently and their behaviour cannot be influenced by the player. Many missions take place inside the space ship, but also often outside. There are a few stealth segments where all weapons are removed and no sound can be made while walking through a large amount of aliens.
Opponents include famous xenomorph aliens from the first three films, such as the chestburster, drone, facehugger, queen, warrior and runner. There are also new types, such as a boiler that can explode and a crusher. Certain levels include encounters with human soldiers from the Weyland-Yutani corporation with a different interest in the colony. There are also a number of boss fights.
Multiplayer is available as a cooperative campaign for two (split-screen) or four (online) players. Competitive multiplayer modes include Escape, Extermination, Survivor and Team Deathmatch. Escape and Survivor draw inspiration for the Left 4 Dead series. In the first, four human players need to reach a destination, while in the second they need to barricade themselves and survive as long as possible. In both modes both sides (humans and aliens) are controlled by human players. Playing as xenomorphs, there is a large emphasis on using the environment to travel along walls, ceilings and through vents for unexpected attacks. Different types of aliens are available and these too have different customization options and attacks unlocked as the player levels up.