Natural Selection is one of the most popular and critically acclaimed Half-Life mods. Set in the future, Natural Selection (NS) is a multiplayer modification that tells the story of a struggle between two forces: the Trans-System Authority (TSA) and the alien Kharaa. The TSA is comprised of marines, led by a commander who leads his team to victory. The Kharaa are a group of five life forms, each with a specific skill set designed to compliment the team. Natural Selection has two different game modes: Classic and Combat.

      NS Classic is a FPS/RTS hybrid, which means the gameplay has aspects of both a first-person shooter (like Half-Life) and a real-time strategy game (like Starcraft). The marines, armed with guns, mines, grenades, and other weapons, follow the orders of their commander. The commander is a player on the marine team who views the action from above, and is able to guide his team by issuing orders and creating buildings for the other players to build. The Kharaa have five different life forms, and while they have no commander they still need to work together to defeat their foe. Each life form has different abilities and is useful in a different way: a Skulk is a mobile scout, while a Gorge is a builder and an Onos is adept at assaulting the enemy.

      Each team secures resource nodes by constructing a building on them. Once a node is acquired, it will provide resources for the team. The TSA commander uses these resources to make buildings, create weapons and armor for his soldiers, or to upgrade his soldier’s offensive and defensive capabilities. The Kharaa use the resources to construct their own buildings, to evolve different features such as improved armor, and to change into different life forms.

      In order to win, the marines must destroy all the alien hives, up to three of which can be constructed. The aliens must kill all the marines, which normally involves eliminating the marine buildings in their base.

      NS Combat is the second game mode. This is a faster paced game mode, focused more on action than on strategy. The marines work as a team to destroy the single Kharaa hive, and the aliens must destroy them marine command console. There is no commander in this mode, and each player, marine or alien, earns experience points for killing an opponent. These points are spent like resources are in Classic mode, and are used to purchase upgrades or new equipment.