T-Mobile is a group of mobile phone corporate subsidiaries (all under the ownership of Deutsche Telekom) that operate GSM networks in Europe and the United States. The "T" stands for "Telekom." Most subsidiaries of Deutsche Telekom have names beginning with "T-". T-Mobile also has financial stakes in mobile operators in Eastern Europe. Globally, T-Mobile has 101 million subscribers, making it the world's sixth largest mobile phone service provider by subscribers and the third largest multinational after the United Kingdom's Vodafone and Spain's Telefonica.

T-Mobile has a substantial presence in ten European countries: Austria, Croatia, Czech Republic, Germany, Hungary, the Republic of Macedonia, the Netherlands, Poland, Slovakia and the United Kingdom as well as in the United States.

Deutsche Telekom recently attempted to acquire rival mobile network operator O2, but it was eventually acquired by Spain's Telefonica.

As well as kit sponsors for FC Bayern Munich, Ferencvárosi TC and West Bromwich Albion, T-Mobile is also a sponsor of several sports events, some of which carry the company name. For example, it sponsors its own cycling team, the T-Mobile Team, and the Austrian first division football competition, the T-Mobile Bundesliga. It was also the official global mobile phone carrier for the 2006 FIFA World Cup (football) in Germany.

T-Mobile USA was previously known as VoiceStream Wireless (formerly a division of Western Wireless until it was spun off as an independent company in 1999 which by the end of that year had acquired regional GSM carriers Aerial Communications in the Midwest and Omnipoint in the Northeast). In May 2001, VoiceStream, along with Southern regional carrier Powertel were acquired by Deutsche Telekom for $24 billion, and changed nationally to the T-Mobile name in September 2002. Headquartered in Factoria, Bellevue, Washington, T-Mobile USA is currently the fourth-largest wireless carrier in the U.S. market with 27.5 million customers (as of Q4 2006). As the smallest national carrier, T-Mobile USA tends to compete on price, and advertises heavily to the youth market. They also occasionally decide to compete on features; they were the first U.S carrier to launch a ringback tones service nationally ("CallerTunes"), video messaging and "HiFi Ringers" (ringtones which are clips of an actual song). T-Mobile USA's corporate slogan is "Stick Together", recently changed from "Get More" on October 1, 2006.

In addition to mobile phone service, T-Mobile also operates WiFi hotspots in the United States and Europe. In the USA alone it operates over 8,000 (22 December 2006) T-Mobile HotSpot locations for Internet access - including such locations as airports, airline clubs, Starbucks coffeehouses, Kinko's, Borders Books and Music, Hyatt and Red Roof Inn Hotels. The US WiFi infrastructure was completely replaced when T-Mobile bought the Wireless ISP MobileStar.

On November 28, 2006 Nokia Corp. of Finland and Telefon AB LM Ericsson of Sweden and announced they will provide 3G mobile phone infrastructure to T-Mobile USA. The financial terms of the contract weren't disclosed. In October of 2006 T-Mobile announced it will spend $2.75 billion over the next few years to upgrade to its next generation 3G network.