T-Mobile USA, Inc. is an American mobile-network operator, headquartered in Bellevue, Washington, that provides wireless voice, messaging and data services in the United States, Puerto Rico and the U.S. Virgin Islands. The company is the fourth-largest wireless carrier in the U.S. market with 33.73 million customers and annual revenues of US$21.35 billion. As of 2011^[update], J.D. Power and Associates, a global marketing-information-services firm, ranked the company highest among major wireless carriers for retail-store satisfaction four years consecutively and highest for wireless customer care two years consecutively.

The company traces its roots to the 1994 establishment of VoiceStream Wireless Wireless PCS, originally a subsidiary of Western Wireless Corporation. Western Wireless spun off VoiceStream Wireless to shareholders in 1999, creating a public independent company, VoiceStream Wireless Corporation. In July 2002, VoiceStream Wireless Corporation was renamed T-Mobile USA, Inc. which operates as the U.S. operating entity of T-Mobile International AG, the mobile-communications holding company and subsidiary of Deutsche Telekom.

T-Mobile International AG is a German holding company for Deutsche Telekom AG's various mobile communications subsidiaries outside Germany. Based in Bonn, Germany, its subsidiaries operate GSM, UMTS and LTE-based cellular networks in Europe, the United States, Puerto Rico and the US Virgin Islands. The company has financial stakes in mobile operators in both Central and Eastern Europe.

The T-Mobile brand is present in 11 European countries – Austria, Croatia, Czech Republic, Germany, Hungary, Macedonia, Montenegro, the Netherlands, Poland, Slovakia, and the United Kingdom – as well as the US, Puerto Rico and the US Virgin Islands.

Globally, T-Mobile International subsidiaries have a combined total of approximately 150 million subscribers, making the company the world's third-largest mobile-phone service provider by subscribers and the third-largest multinational after the UK's Vodafone and Spain's Telefónica.

Germany's initial mobile-communications services were radiotelephone systems that were owned and operated by the state postal monopoly, Deutsche Bundespost. It launched the analog first-generation C-Netz ("C Network", marketed as C-Tel), Germany's first true mobile phone network in 1985.