SMS (short message service) is a text messaging service component of most telephone, internet, and mobile-device systems. It uses standardised communication protocols to enable mobile devices to exchange short text messages. An intermediary service can facilitate a text-to-voice conversion to be sent to landlines. SMS was the most widely used data application, with an estimated 3.5 billion active users, or about 80% of all mobile subscribers, at the end of 2010.

SMS, as used on modern devices, originated from radio telegraphy in radio memo pagers that used standardised phone protocols. These were defined in 1985 as part of the Global System for Mobile Communications (GSM) series of standards and is referred to under the 3GPP TS 23.040 specification. Although most SMS messages are mobile-to-mobile text messages, support for the service has expanded to include other mobile technologies, such as ANSI CDMA networks and Digital AMPS.

SMS is also employed in mobile marketing, a type of direct marketing. According to one market research report, as of 2014, the global SMS messaging business was estimated to be worth over $100 billion, accounting for almost 50 percent of all the revenue generated by mobile messaging.

SMS in use on GSM mobile networks supports up to 160 characters when using the GSM default alphabet or 70 UCS2 characters. When a message is longer than these limits then a "long SMS" can be sent that uses segmentation and reassembly (SAR) to split the long message into multiple SMS message parts that use meta data to associate the parts.

SMS can be sent from one mobile to another mobile. The sending mobile passes the SMS to their own network's Short Message Service Center (SMSC) and this stage is known as the Mobile Originated (MO) SMS. The SMSC then attempts delivery of the message to the destination mobile, which may be on the same network or another interconnected mobile network. This latter stage is known as Mobile Terminated (MT) SMS.