Natural Selection 2 is the sequel to the Half-Life mod Natural Selection. The game is a team based shooter with a twist. Two teams vie for control of futuristic locations such as off-world mining sites or high-tech bases. One team consists of marines the other of an alien life force. The goal for both teams is to destroy all opposing bases (up to three). Marines get an arsenal of weaponry including mines, shotguns, jetpacks, and battlesuits with rail or miniguns. The aliens on the other hand rely mostly on biting, slashing and stomping, but can rely on greater speed and stealth capabilities to defeat their foe.
Both teams are led by a commander who in effect plays a Real Time Strategy game with the other players in his team as his units. Commanders see the game in a top-down view rather than the first person view that the other players enjoy. In the case of the marines the commander can order the construction of buildings, which must be completed by its marines. Specific buildings allow for the marines to get certain weapons and for the commander to upgrade them and construct some NPC units such as a healing robot and artillery. The marine commander can also drop medpacks and ammunition to help out marines in combat. The alien commander can construct buildings manually without having to rely on his team mates. Like the Zerg in StarCraft the aliens can only build on infestation which spreads slowly through the construction of cysts. Because the aliens don't have to help their commander as much as the marines and have quicker movement they have more time to harass the marines.
Throughout the maps there are various resource nozzles which can be captured by both aliens and marines. These nozzles allow for the commanders to build their structures, but other players use them as well. The marines use them to purchase weapons and aliens use them to evolve to different species. The aliens have five different forms: the Skulk which is fast and can run on walls, the Gorge which can heal, build walls and tunnels, the Lerk which can fly, the Fade which can blink quickly to get behind the enemy, and the Onos which is the most powerful and most expensive.