MS-DOS (abbreviation for Microsoft Disk Operating System) was a command-line driven operating system family sold as a standalone product in the 1980's and early 1990's, later evolving to be used as a platform on which to run a shell that would later be known as Microsoft Windows.

In 1980, Microsoft then-head Bill Gates (known for his creation of the BASIC computer programming language) was approached by IBM about an operating system for their computer boxes. Paul Allen contacted Tim Patterson of Seattle Computer Products, and acquired the rights to sell his QDOS (Quick and Dirty Operating System) to an unnamed client - IBM - for under $100,000. Deals continued as the product was still under development at Seattle Computer Products; in 1981 Microsoft bought the full rights to the operating system, which they trademarked as MS-DOS.

Legions of games were developed and marketed as being compatible with MS-DOS, and thus the platform is still fairly important, if somewhat ancient. Many of these games can run on FreeDOS, or within DOSBox.