The history of macOS, Apple's current Mac operating system (formerly named Mac OS X until 2011 and then OS X until 2016), began with the company's project to replace its classic Mac OS. That system, up to and including its final release Mac OS 9, was a direct descendant of the operating system Apple had used in its Mac computers since their introduction in 1984. The current macOS is a UNIX operating system built on technology that had been developed at NeXT from the 1980s until Apple purchased the company in early 1997. macOS components derived from BSD include multiuser access, TCP/IP networking, and memory protection. Although it was originally marketed as simply "version 10" of Mac OS (indicated by the Roman numeral "X"), it has a completely different codebase from Mac OS 9, as well as substantial changes to its user interface (UI). The transition was a technologically and strategically significant one. To ease the transition for users and developers, versions 10.0 through 10.4 were able to run Mac OS 9 and its applications in the Classic Environment, a compatibility layer. macOS was first released in 1999 as Mac OS X Server 1.0, built using the technologies Apple acquired from NeXT, but did not include the signature Aqua UI. Mac OS X 10.0 is the first desktop version, aimed at regular users, released in March 2001. Several more distinct desktop and server editions of macOS have been released since. Mac OS X Server is no longer offered as a standalone operating system with the release of Mac OS X 10.7 Lion. Instead, server management tools were provided as an application, available as a separate add-on, until it was discontinued on April 21, 2022, making it incompatible with macOS 13 Ventura or later. Releases of macOS, starting with the Intel build of Mac OS X 10.5 Leopard, are certified as Unix systems conforming to the Single UNIX Specification. Mac OS X Lion was the first release for which the shortened OS X name was occasionally used, where it was sometimes called OS X Lion, but this naming scheme was first officially adopted as the sole branding with OS X Mountain Lion. The operating system was further renamed macOS with the release of macOS Sierra. Mac OS X 10.0 and 10.1 were given names of big cats as internal code names, Cheetah and Puma. Starting with Mac OS X 10.2 Jaguar, big-cat names were used as marketing names. Beginning with OS X 10.9 Mavericks, names of locations in California were used as marketing names instead. macOS retained the major version number 10 throughout its development history until the release of macOS 11 Big Sur in 2020, where its major version number was incremented by one with each release. In 2025, Apple unified the versioning across all products, including its other operating systems, to match the year after its WWDC announcement, beginning with macOS 26 Tahoe, which was released on September 15, 2025.
Development
Development outside Apple
After Apple removed Steve Jobs from management in 1985, he left the company and, with funding from Ross Perot, founded NeXT, Inc. The result was the NeXT Computer. As the first workstation to include a digital signal processor (DSP) and a high-capacity optical disc drive, NeXT hardware was advanced for its time, but was expensive relative to the rapidly commoditizing workstation market. Although the hardware was phased out in 1993, the company's object-oriented operating system, NeXTSTEP, had a more lasting legacy as it eventually became the basis for Mac OS X. NeXTSTEP was based on the Mach kernel developed at Carnegie Mellon University (CMU) and BSD, an implementation of Unix dating back to the 1970s. It featured an object-oriented programming framework based on the Objective-C language. This environment is known today in the Mac world as Cocoa. It also supported the innovative Enterprise Objects Framework database access layer and WebObjects application server development environment, among other notable features. All but abandoning the idea of an operating system, NeXT managed to maintain a business selling WebObjects and consulting services, only ever making modest profits in its last few quarters as an independent company. NeXTSTEP underwent an evolution into OPENSTEP, which separated the object layers from the operating system below, allowing it to run with less modification on other platforms. OPENSTEP was, for a short time, adopted by Sun and HP. By this point, however, a number of other companies — notably Apple, IBM, Microsoft, and even Sun itself — were claiming they would soon be releasing similar object-oriented operating systems and development tools of their own. Some of these efforts, such as Taligent, did not fully come to fruition; others, like Java, gained widespread adoption. On February 4, 1997, Apple Computer acquired NeXT for $427 million, and used OPENSTEP as the basis for Mac OS X, as it was called at the time. Traces of the NeXT software heritage can still be seen in macOS. For example, in the Cocoa development environment, the Objective-C library classes have "NS" prefixes, and the HISTORY section of the manual page for the defaults command in macOS straightforwardly states that the command "First appeared in NeXTStep."
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