NeXTSTEP was the operating system and development environment built by NeXT, the company Steve Jobs founded after leaving Apple in 1985. It combined a Unix foundation (derived from BSD and the Mach microkernel) with a graphical interface rendered by Display PostScript and a set of object-oriented application frameworks written in Objective-C. The first release shipped on the NeXT Computer, and the system matured through Release 3.x in the early 1990s, by which point its developer documentation, archived in the bitsavers collection at the Internet Archive, ran to a full library of manuals covering the user interface, the Application Kit, and the programming interface summary.
What made NeXTSTEP distinctive was not its hardware but its software model. The “NeXTSTEP User Interface Guidelines Release 3” describes a consistent, object-based way of constructing applications, and the companion “NeXTSTEP Programming Interface Summary” catalogs the classes a developer worked with. Programs were assembled from reusable Objective-C objects rather than written from scratch, and the system shipped with Interface Builder, a visual tool that let developers lay out windows and wire up connections between objects graphically instead of by hand.
The Computer History Museum, in its history of the platform, describes how Jobs founded NeXT in 1985 and how, although “the hardware proved too expensive,” NeXTSTEP “became innovative and productive” as a development platform. The same account notes that Objective-C, created by Brad Cox in the 1980s to add Smalltalk-style object orientation to C, “became the programming foundation for NeXTSTEP and remains in use at Apple today.”
NeXT also reworked NeXTSTEP’s frameworks into a portable specification called OpenStep, developed jointly with Sun Microsystems, which separated the object libraries (Foundation and the Application Kit) from the underlying operating system. That separation is why the same APIs could later be carried onto entirely different kernels and hardware.
The decisive moment came in December 1996, when Apple acquired NeXT and brought Jobs back with it. NeXTSTEP’s frameworks were renamed and became Cocoa, its Foundation and Application Kit libraries forming the core of Mac OS X. When the iPhone shipped in 2007, that same lineage carried forward into iOS through Cocoa Touch and UIKit. The class-name prefix “NS,” still visible across Apple’s Foundation framework decades later, is a direct fossil of NeXTSTEP.