Microsoft Windows is the name of several families of proprietary software operating systems by Microsoft.

Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces (GUI).

Microsoft Windows eventually came to dominate the world's personal computer market, overtaking OS/2 and Mac OS which had been introduced earlier. At the 2004 IDC Directions conference, IDC Vice President Avneesh Saxena stated that Windows had approximately 90% of the client operating system market.