MCI Communications Corp. was an American telecommunications company that was instrumental in legal and regulatory changes that led to the breakup of the AT&T monopoly of American telephony and ushered in the competitive long-distance telephone industry. It was headquartered in Washington, D.C.

Founded in 1963, it grew to be the second-largest long-distance provider in the U.S. It was purchased by WorldCom in 1998 and became MCI WorldCom, with the name afterwards being shortened to WorldCom in 2000. WorldCom's financial scandals and bankruptcy led that company to change its name in 2003 to MCI. The MCI name disappeared in January 2006 after the company was bought by Verizon. As of May 2011, the MCI trademark is still maintained on MCI.com and on a sub-page of Verizon.com.

MCI was founded as Microwave Communications, Inc. on 3 October 1963 with John D. Goeken being named the company's first president. The initial business plan was for the company to build a series of microwave relay stations between Chicago, Illinois and St. Louis, Missouri. The relay stations would then be used to interface with limited-range two-way radios used by truckers along U.S. Route 66 or by barges on the Illinois Waterway. The long-distance communication service would then be marketed to shipping companies that were too small to build their own private relay systems. In addition to the radio relay services, MCI soon made plans to offer voice, computer information, and data communication services for business customers unable to afford AT&T's TELPAK service.