United States v. Microsoft Corp.: Microsoft is ruled to have violated United States antitrust law by keeping "an oppressive thumb" on its competitors.

United States of America v. Microsoft Corporation, 253 F.3d 34, was a landmark American antitrust law case at the United States Court of Appeals for the District of Columbia Circuit. The U.S. government accused Microsoft of illegally monopolizing the web browser market for Windows, primarily through the legal and technical restrictions it put on the abilities of PC manufacturers (OEMs) and users to uninstall Internet Explorer and use other programs such as Netscape and Java.

Microsoft Corporation is an American multinational corporation and technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the rise of personal computers through software like Windows, and the company has since expanded to Internet services, cloud computing, video gaming and other fields. Microsoft is the largest software maker, one of the most valuable public U.S. companies, and one of the most valuable brands globally.

In the United States, antitrust law is a collection of mostly federal laws that govern the conduct and organization of businesses in order to promote economic competition and prevent unjustified monopolies. The three main U.S. antitrust statutes are the Sherman Act of 1890, the Clayton Act of 1914, and the Federal Trade Commission Act of 1914. Section 1 of the Sherman Act prohibits price fixing and the operation of cartels, and prohibits other collusive practices that unreasonably restrain trade. Section 2 of the Sherman Act prohibits monopolization. Section 7 of the Clayton Act restricts the mergers and acquisitions of organizations that may substantially lessen competition or tend to create a monopoly. The Robinson–Patman Act, an amendment to the Clayton Act, prohibits price discrimination.