Microsoft Flight Simulator (often abbreviated as MSFS or FS) is a series of amateur flight simulator programs for Microsoft Windows operating systems, and earlier for MS-DOS and Classic Mac OS. It is one of the longest-running, best-known, and most comprehensive home flight simulator programs on the market. It was an early product in the Microsoft application portfolio and differed significantly from Microsoft's other software, which was largely business-oriented. At 38 years old, it is the longest-running software product line for Microsoft, predating Windows by three years. Microsoft Flight Simulator is one of the longest-running PC video game series of all time.