The Macintosh operating system was designed at Apple in the early 1980s, inspired by pioneering work on GUIs done earlier at Xerox's Palo Alto Research Center. It saw its debut with the Macintosh in 1984. MacOS has gone through two significant design transitions since, and is undergoing a third. The first transition was the shift from supporting only a single application at a time to being able to cooperatively multitask multiple applications (MultiFinder); the second was the shift from 68000 to PowerPC processors, which both preserved backward binary compatibility with 68K applications and brought in an advanced shared library management system for PowerPC applications, replacing the original 68K trap instruction-based code-sharing system. The third was the merger of MacOS design ideas with a Unix-derived infrastructure in MacOS X. Except where specifically noted, the discussion here applies to pre-OS-X versions.

MacOS has a very strong unifying idea that is very different from Unix's: the Mac Interface Guidelines. These specify in great detail what an application GUI should look like and how it should behave. The consistency of the Guidelines influenced the culture of Mac users in significant ways. Not infrequently, simple-minded ports of DOS or Unix programs that did not follow the Guidelines have been summarily rejected by the Mac user base and failed in the marketplace.

One key idea of the Guidelines is that things stay where you put them. Documents, directories, and other objects have persistent locations on the desktop that the system doesn't mess with, and the desktop context persists through reboots.

The Macintosh's unifying idea is so strong that most of the other design choices we discussed above are either forced by it or invisible. All programs have GUIs. There is no CLI at all. Scripting facilities are present but much less commonly used than under Unix; many Mac programmers never learn them. MacOS's captive-interface GUI metaphor (organized around a single main event loop) leads to a weak scheduler without preemption. The weak scheduler, and the fact that all MultiFinder applications run in a single large address space, implies that it is not practical to use separated processes or even threads rather than polling.

MacOS applications are not, however, invariably monster monoliths. The system's GUI support code, which is partly implemented in a ROM shipped with the hardware and partly implemented in shared libraries, communicates with MacOS programs through an event interface that has been quite stable since its beginnings. Thus, the design of the operating system encourages a relatively clean separation between application engine and GUI interface.

MacOS also has strong support for isolating application metadata like menu structures from the engine code. MacOS files have both a ‘data fork’ (a Unix-style bag of bytes that contains a document or program code) and a ‘resource fork’ (a set of user-definable file attributes). Mac applications tend to be designed so that (for example) the images and sound used in them are stored in the resource fork and can be modified separately from the application code.

The MacOS system of internal boundaries is very weak. There is a wired-in assumption that there is but a single user, so there are no per-user privilege groups. Multitasking is cooperative, not pre-emptive. All MultiFinder applications run in the same address space, so bad code in any application can corrupt anything outside the operating system's low-level kernel. Security cracks against MacOS machines are very easy to write; the OS has been spared an epidemic mainly because very few people are motivated to crack it.