MS-DOS (Microsoft Disk Operating System) is an operating system commercialized by Microsoft. It was the commonly used member of the DOS family of operating systems and was the dominant operating system for the PC compatible platform during the 1980s. It has gradually been replaced on consumer desktop computers by a variety of generations of the Windows operating system.

MS-DOS was initially released in 1981 and had eight major versions released before Microsoft stopped development in 2000. It was the key product in Microsoft's growth from a programming languages company to varied software development firm, providing the company with essential revenue and marketing resources.