Microsoft has been involved in numerous high-profile legal matters that involved litigation over the history of the company, including cases against the United States, the European Union, and competitors.

In its 2008 annual report, Microsoft stated:

Government regulatory actions and court decisions may hinder our ability to provide the benefits of our software to consumers and businesses, thereby reducing the attractiveness of our products and the revenues that come from them. New actions could be initiated at any time, either by these or other governments or private claimants, including with respect to new versions of Windows or other Microsoft products. The outcome of such actions, or steps taken to avoid them, could adversely affect us in a variety of ways, including:

In the 1990s, Microsoft adopted exclusionary licensing under which PC manufacturers were required to pay for an MS-DOS license even when the system was shipped with an alternative operating system. Critics attest that it also used predatory tactics to price its competitors out of the market and that Microsoft erected technical barriers to make it appear that competing products did not work on its operating system. In a consent decree filed on July 15, 1994, Microsoft agreed to a deal under which, among other things, the company would not make the sale of its operating systems conditional on the purchase of any other Microsoft product. On February 14, 1995, Judge Stanley Sporkin issued a 45-page opinion that the consent decree was not in the public interest. Later that spring, a three-judge federal appeals panel removed Sporkin and reassigned the consent decree. Judge Thomas Penfield Jackson entered the decree on August 21, 1995, three days before the launch of Windows 95.

A Microsoft purchase of Intuit was scuttled in 1994 due to antitrust concerns that Microsoft would be purchasing a major competitor.

After bundling the Internet Explorer web browser into its Windows operating system in the late 1990s (without requiring a separate purchase) and acquiring a dominant share in the web browser market, the antitrust case United States v. Microsoft was brought against the company. In a series of rulings by judge Thomas Penfield Jackson, the company was found to have violated its earlier consent decree and abused its monopoly in the desktop operating systems market. The "findings of fact" during the antitrust case established that Microsoft has a monopoly in the PC desktop operating systems market:

Viewed together, three main facts indicate that Microsoft enjoys monopoly power. First, Microsoft's share of the market for Intel-compatible PC operating systems is extremely large and stable. Second, Microsoft's dominant market share is protected by a high barrier to entry. Third, and largely as a result of that barrier, Microsoft's customers lack a commercially viable alternative to Windows. (III.34)

The findings of fact go on to explain the nature of the "barrier to entry":