Mac OS X was based on technology developed at Steve Jobs’s NeXT. Apple used NeXTSTEP and the related OPENSTEP technology as the foundation for its next-generation operating system.
After Apple acquired NeXT in 1997, Jobs returned as an adviser and later became interim chief executive. The NeXT acquisition supplied Apple with a modern operating-system architecture based on the Mach kernel and BSD Unix components.
Mac OS X was introduced in 2001, replacing the older classic Mac OS over time. Its underlying technologies also influenced Apple’s mobile software. iPhone OS, later renamed iOS, shared important foundations with Mac OS X, while receiving an interface and software framework designed for touch-based devices.