AT&T Inc. (originally the American Telephone and Telegraph Company) is an American multinational conglomerate holding company that is Delaware-registered but headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company and the largest provider of mobile telephone services in the U.S. As of 2020, AT&T was ranked 9th on the Fortune 500 rankings of the largest United States corporations, with revenues of $181 billion.During most of the 20th century, AT&T had a monopoly on phone service in the United States. The company began its history as the American District Telegraph Company, formed in St. Louis in 1878. After expanding services to Arkansas, Kansas, Oklahoma and Texas, through a series of mergers, it became Southwestern Bell Telephone Company in 1920, which was then a subsidiary of American Telephone and Telegraph Company. The latter was a successor of the original Bell Telephone Company founded by Alexander Graham Bell in 1877. The American Bell Telephone Company formed the American Telephone and Telegraph Company (AT&T) subsidiary in 1885. In 1899, AT&T became the parent company after the American Bell Telephone Company sold its assets to its subsidiary. The company was rebranded as AT&T Corp. in 1994. The 1982 United States v. AT&T antitrust lawsuit resulted in the divestiture of AT&T's ("Ma Bell") local operating subsidiaries which were grouped into seven Regional Bell Operating Companies (RBOCs), commonly referred to as "Baby Bells", resulting in seven independent companies, including Southwestern Bell Corporation (SBC). The latter changed its name to SBC Communications Inc. in 1995.In 2005, SBC purchased its former parent AT&T Corp. and took on its branding, with the merged entity naming itself AT&T Inc. and using its history, a version of its iconic logo and stock-trading symbol which launched on December 30, 2005. AT&T Inc. acquired BellSouth Corporation in 2006, the last independent Baby Bell company, making its formerly joint venture Cingular Wireless (which had acquired AT&T Wireless in 2004) wholly owned and rebranding it as AT&T Mobility. AT&T Inc. also acquired Time Warner in 2016, with the proposed merger confirming on June 12, 2018 and the aim of making AT&T the largest and controlling shareholder of Time Warner and rebranding it as WarnerMedia in 2018.

The current AT&T reconstitutes much of the former Bell System, and includes four of the seven "Baby Bells" along with the original AT&T Corp., including the long-distance division.