Natural Selection blends first-person combat with real-time strategy. It features intense "marines vs. aliens" teamplay, where one marine can become Commander. The Commander plays from a top-down perspective and leads live players. Aliens, on the other hand, can mutate into more powerful or useful forms.

NS has atmospheric indoor sci-fi environments full of steam and smoke, and features custom orchestral music from the award-winning Jeremy Soule.

It is a freely-available Half-life modification, and since its release in October of 2002, has had over 2 million downloads, and has 10,000 unique players every day.