Over the next two years, major effort was applied to porting the original Macintosh APIs to Unix libraries known as Carbon.

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 Macintosh computers since their introduction in 1984.

However, 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.

The transition was a technologically and strategically significant one.

To ease the transition, versions through 10.4 were able to run Mac OS 9 and its applications in a compatibility layer.

Starting with Mac OS X 10.7 Lion, macOS Server is no longer offered as a separate operating system; instead, server management tools are available for purchase as an add-on.

Starting with the Intel build of Mac OS X 10.5 Leopard, most releases have been certified as Unix systems conforming to the Single Unix Specification.

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 and marred by design problems.

It featured an object-oriented programming framework based on the Objective-C language.

It also supported the innovative Enterprise Objects Framework database access layer and WebObjects application server development environment, among other notable features.

OPENSTEP was, for a short time, adopted by Sun Microsystems.

However, by this point, 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.).

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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.

The decade-old Macintosh System Software had reached the limits of its single-user, co-operative multitasking architecture, and its once-innovative user interface was looking increasingly outdated.

A massive development effort to replace it, known as Copland, was started in 1994, but was generally perceived outside Apple to be a hopeless case due to political infighting and conflicting goals.

By 1996, Copland was nowhere near ready for release, and the project was eventually cancelled.

Some elements of Copland were incorporated into Mac OS 8, released on July 26, 1997.

Avie Tevanian took over OS development, and Steve Jobs was brought on as a consultant.

At first, the plan was to develop a new operating system based almost entirely on an updated version of OPENSTEP, with the addition of a virtual machine subsystem known as the Blue Box for running classic Macintosh applications.

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The result was known by the code name Rhapsody, slated for release in late 1998.

Instead, several major developers such as Adobe told Apple that this would never occur, and that they would rather leave the platform entirely.

This rejection of Apples plan was largely the result of a string of previous broken promises from Apple; after watching one next OS after another disappear and Apples market share dwindle, developers were not interested in doing much work on the platform at all, let alone a re-write.

The board asked Steve Jobs to lead the company on an interim basis, essentially giving him carte blanche to make changes to return the company to profitability.

When Jobs announced at the World Wide Developers Conference that what developers really wanted was a modern version of the Mac OS, and Apple was going to deliver it citation needed, he was met with thunderous applause.

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