MCI, Inc. (subsequently Worldcom and MCI WorldCom) was a telecommunications company. For a time, it was the second largest long-distance telephone company in the United States, after AT&T. Worldcom grew largely by acquiring other telecommunications companies, including MCI Communications in 1998, and filed bankruptcy in 2002 after an accounting scandal, in which several executives, including CEO Bernard Ebbers, were convicted of a scheme to inflate the company's assets. In January 2006, the company, by then renamed MCI, was acquired by Verizon Communications and was later integrated into Verizon Business.

Worldcom was originally headquartered in Clinton, Mississippi before relocating to Ashburn, Virginia when it changed its name to MCI.

MCI Communications Corp. (originally Microwave Communications, Inc.) was a telecommunications company headquartered in Washington, D.C. that was at one point the second-largest long-distance provider in the United States.

MCI was instrumental in legal and regulatory changes that led to the breakup of the monopoly of AT&T Corporation and introduced competition in the telephone industry. Its MCI Mail, launched in 1983, was one of the first Email services and its MCI.net was an integral part of the Internet backbone.

The company was acquired by WorldCom (later called MCI Inc.) in 1998.