Unstructured Supplementary Service Data (USSD) is a technology that makes communication interactive and easy to use. It can be accessed from a simple GSM mobile phone, with no special application requirements on the handset.

Each USSD session is either initiated by the user or the mobile operator. The user is asked to choose between various options provided. The progress of one session into the next depends on the user's response. USSD enables a variety of applications on the mobile phone, for example enabling roaming on a pre-paid connection, initiating a chat session, finding a friend in the network etc.