MCI, Inc. was an American telecommunications company that was headquartered in Ashburn, Virginia. The corporation was the result of the merger of WorldCom (formerly known as LDDS followed by LDDS WorldCom) and MCI Communications, and used the name MCI WorldCom followed by WorldCom before taking its final name on April 14, 2003 as part of the corporation's emergence from bankruptcy. The company formerly traded on NASDAQ under the symbols "WCOM" (pre-bankruptcy) and "MCIP" (post-bankruptcy). The corporation was purchased by Verizon Communications with the deal closing on January 6, 2006, and is now identified as that company's Verizon Business division with the local residential divisions slowly integrated into local Verizon subsidiaries.

MCI's history, combined with the histories of companies it has acquired, echoes most of the trends that have swept American telecommunications in the past half-century: It was instrumental in pushing legal and regulatory changes that led to the breakup of the AT&T monopoly that dominated American telephony; its purchase by WorldCom and subsequent bankruptcy in the face of accounting scandals was symptomatic of the Internet excesses of the late 1990s. It accepted a proposed purchase by Verizon for US$7.6 billion.

For a time, WorldCom (WCOM) was the United States' second largest long distance phone company (AT&T was the largest). WorldCom grew largely by acquiring other telecommunications companies, most notably MCI Communications. It also owned the Tier 1 ISP UUNET, a major part of the Internet backbone. It was based in Clinton, Mississippi before moving to Ashburn, Virginia.