The Counter-Strike series is a series of multiplayer first-person shooter video games primarily developed for Microsoft Windows and later, the Mac OS X, by Valve Corporation. Counter-Strike was later ported to the Xbox while Counter-Strike: Global Offensive has been released for Microsoft Windows, Mac OS X, PlayStation 3, and Xbox 360—a first for the series. The series has been known for its competitive, challenging gameplay, its high use of team work and tactics, and the use of the GoldSrc and Source engines, known for their accessibility and being friendly for game modification.