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.

The aliens can evolve upgrades like "Cloaking" and "Scent of Fear", and can change lifeforms to run on walls, fly, and even devour marines whole.

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.