AT&T developed Unix through researchers at its Bell Labs subsidiary.
Unix originated at Bell Labs, then part of AT&T, in the late 1960s and early 1970s. Ken Thompson, Dennis Ritchie, and other researchers created the system after work on the abandoned Multics project. Thompson wrote an early version, while Ritchie later created the C programming language, which helped make Unix portable across different hardware.
Bell Labs distributed Unix to universities and research institutions, where it influenced generations of operating systems and software developers. Its design principles, including small tools that could be combined through pipelines, became especially influential. Unix descendants and relatives include BSD systems, macOS's underlying technologies, and many commercial Unix variants.
AT&T was the dominant American telephone company for much of the twentieth century. A U.S. antitrust settlement broke up the Bell System in 1984, separating regional operating companies from AT&T. The company later re-entered telecommunications through mergers and acquisitions. AT&T's market value has varied substantially over time, but its role in Unix history is tied specifically to Bell Labs.