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The Z-machine is a virtual machine that was developed by Joel Berez and Marc Blank in 1979 and used by Infocom for its text adventure games. Infocom compiled game code to files containing Z-machine instructions (called story files, or Z-code files), and could therefore port all its text adventures to a new platform simply by writing a Z-machine implementation for that platform. With the large number of incompatible home computer systems in use at the time, this was an important advantage over using native code (or developing some compiler for each system).

The "Z" of Z-machine stands for Zork, Infocom's first adventure game. Z-code files usually have names ending in .z1, .z2, .z3, .z4, .z5, .z6, .z7 or .z8 (and occasionally .dat), where the number is the version number of the Z-machine on which the file is intended to be run, as given by the first byte of the story file. Previously it was common for the filenames to end with .zip (ZIP = Z-machine Interpreter Program), but this clashes with the present widespread use of .zip for PKZIP-compatible archive files. Infocom produced six versions of the Z-machine. Files using versions 1 and 2 are very rare. Only two version 1 files are known to have been released by Infocom, and only two of version 2. The later versions had more capabilities, culminating in some graphic support in version 6.

The compiler (called Zilch) which Infocom used to produce its story files has never been released, although documentation of the language used (called ZIL, for Zork Implementation Language) is still in existence. But in May 1993, Graham Nelson released the first version of his Inform compiler, which also generates Z-machine story files as its output, even though the Inform source language is quite different from ZIL. Most files produced by Inform are version 5.

Inform has since become very popular in the interactive fiction community and, as a consequence, a large proportion of the interactive fiction now produced is in the form of Z-machine story files. Demand for the ability to create larger game files led Graham Nelson to specify versions 7 and 8 of the Z-machine, though version 7 is very rarely used. Because of the way addresses are handled, a version 3 story file can be up to 128K in length, a version 5 story can be up to 256K in length, and a version 8 story can be up to 512k in length. Though these sizes may seem small by today's computing standards, for text-only adventures, these are large enough for very elaborate games.

During the 1990s, Graham Nelson drew up a Z-machine standard, based on detailed studies of the existing Infocom files.

Interpreters for Z-code files are available on a wide variety of platforms. In fact, since interpreters are available for a wide range of platforms - for example, on various old machines (such as the Apple II, TRS-80 and Sinclair), portable machines (such as Palm OS devices and the Nintendo Game Boy) and most modern platforms - it can be claimed that Z-code is one of the most portable computer languages ever invented.

Popular interpreters include Nitfol[1] and Frotz. Nitfol makes use of the glk API, and supports versions 1 through 8 of the Z-machine, including the version 6 graphical Z-machine. Save files are stored in the standard Quetzal save format. Binary files are currently available for several different operating systems, including Macintosh, Linux, MS-DOS, and Windows.

• Frotz - a popular implementation
• Glulx - a similar virtual-machine to Z-machine
• SCUMM - Script Creation Utility for Maniac Mansion by LucasArts, a graphical system similar to Z-machine

• The Z-Machine standards document
• ZIL manual
• Interpreters for numerous platforms
• The website of the compiler Inform
• A VBCC compiler backend targeting the Z-machine
• Putting the Z-machine on a small machine