UUNET Technologies, Inc., formerly UUNET Communications Services, was an American commercial Internet service provider. Founded in 1987, it was one of the first and largest commercial ISPs and one of the early Tier 1 networks. It was based in Northern Virginia. Today, UUNET is an internal brand of Verizon Business (formerly MCI).^{[citation needed]}

Prior to its founding, access to Usenet and e-mail exchange from non-ARPANET sites was accomplished using a cooperative network of systems running the UUCP protocol over POTS lines. During the mid-1980s, growth of this network began to put considerable strain on the resources voluntarily provided by the larger UUCP hubs. This prompted Rick Adams, a system administrator at the Center for Seismic Studies, to explore the possibilities of providing these services commercially as a way to reduce the burden on the existing hubs.

With funding in the form of a loan from Usenix, UUNET Communications Services began operations in 1987 as a non-profit corporation providing Usenet feeds, e-mail exchange, and access to a large repository of software source code and related information. The venture proved successful and shed its non-profit status within two years. At the same time, the company changed its name to UUNET Technologies. In 1990, UUNET launched its AlterNet service, which provided access to an IP backbone independent of the constraints of those operated by the government. That network lives on in a much larger form and serves as the core of a set of products that include access at dial-up and broadband speeds as well as web hosting. UUNET raised $6 Million from Accel Partners, Menlo Ventures, and New Enterprise Associates in 1993 and $8.2 million in 1996 for expanding its network and hiring new executives with experience in marketing.

In the mid-1990s, UUNET was the fastest-growing ISP, outpacing MCI and Sprint. At its peak, Internet traffic was briefly doubling every few months, which translates to 10x growth each year. However, the continuing UUNET claims of such growth (long after it had fallen to lower, albeit still substantial levels) artificially fueled the expectations of the dot-com and telecom companies of the late 1990s, leading to the dot-com bubble and crash in 2000/2001.

In 1996, UUNET was acquired by MFS on 30 April 1996. This was an independent acquisition unrelated to the acquisition of MFS by WorldCom. However, as MFS was a public company and the acquisition made the company a Wall Street darling, it likely influenced WorldCom's decision to pursue MFS.

In 1996, UUNET was acquired by WorldCom on 26 August 1996, as part of WorldCom's purchase of MFS Communications Company.

In 2001, UUNET was fully integrated with WorldCom and the name was dropped from all official documents.

In 2002, the owner of UUNET at that time (WorldCom) filed for what was then the largest Chapter 11 bankruptcy protection in history.