GSM (Global System for Mobile communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 80% of the global mobile market uses the standard. GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system. GSM EDGE is a 3G version of the protocol.

GSM (Global System for Mobile Communications) is a second generation, cellular, digital mobile phone system, and it is one of the world's main 2G wireless standards, having become widely established in Europe and many other countries of the world. GSM operates in the three main frequency bands of 900 MHz, 1800 MHz, and 1900 MHz, which are known respectively as GSM 900, GSM 1800 and GSM 1900. In the USA, GSM is also known as PCS (Personal Communication Service), and it operates in the 1900 MHz frequency band, although GSM is not as common in North America as it is in Europe. For historical reasons the 1800 MHz band is also sometimes referred to as PCN (Personal Communication Network).

Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE).