Darwin is now only available as source code. Minor updates were released as packages that were installed separately. Up to Darwin 8.0.1, Apple released a binary installer (as an ISO image) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 systems as a standalone operating system. In 2000, the core operating system components of Mac OS X were released as open-source software under the Apple Public Source License (APSL) as Darwin the higher-level components, such as the Cocoa and Carbon frameworks, remained closed-source. At the time, interim CEO Steve Jobs alluded to British naturalist Charles Darwin by announcing 'because it's about evolution'.

In 1999, Apple announced it would release the source code for the Mach 2.5 microkernel, BSD Unix 4.4 OS, and the Apache Web server components of Mac OS X Server. This was developed into Rhapsody in 1997, Mac OS X Server 1.0 in 1999, Mac OS X Public Beta in 2000, and Mac OS X 10.0 in 2001. After Apple bought NeXT in 1996, it announced it would base its next operating system on OPENSTEP. The heritage of Darwin began with Unix derivatives supplemented by aspects of NeXT's NeXTSTEP operating system (later, since version 4.0, known as OPENSTEP), first released in 1989. 4.3 Darwin 16 – 19 OS X rebranded into macOS.4.2 Darwin 12–15 Mac OS X rebranded into OS X.4.1 Darwin 0–11 and corresponding Mac OS X releases.

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