Natural Selection is an action-strategy game for the PC. It is a "Half-life modification", which means it is written on top of the Half-life game engine. "Mods" are a new sort of underground game development that enables small teams to create games using the technology of existing games. This means less expensive and lower risk game development, which allows experimentation and innovation. This arrangement also implies that "mods" are made as art, not for making money. When complete, Natural Selection will be released for free to all owners of Half-life, which is required to play Natural Selection.