SDL (Specification and Description Language) is a graphical specification language standardized by ITU (International Telecommunication Union). SDL, defined in Z.100, has been evolving since the first recommendation in 1980. Every fourth year an updated revision of the language standard has been adopted. In 1992 Object Oriented features were included in SDL. The standard from1996, called SDL-96, introduced only minor updates.  The current standard is SDL-2000, and it introduces a number of new features, including exception handling, a new data model, and composite states.

Although SDL is widely used in the telecommunications field, it is not designed specifically for describing telecommunications services. Rather SDL is a general purpose specification language for communication systems and embedded systems.

The graphical notation, the formal semantics, and object-oriented concepts makes SDL a powerful and versatile language both for systems specification and their implementation.

The basis for description of behaviour in SDL is communicating extended finite state machines, represented by processes. A process consists of a number of states and a number of transitions connecting the states.

Communication between processes is done by signal exchange. Signals can be exchanged between two processes in a system or between a process and the environment of the system. The remote procedure and remote variable paradigms for information exchange between entities in an SDL system are also supported.

In addition to the SDL recommendation, a number of recommendations has been defined to complement and ease the use SDL in combination with other notations. E.g. recommendations on Message Sequence Chart (MSC) Z.120, on the combined use of SDL and ASN.1 (Abstract Syntax Notation 1) Z.105, and an layout preserving interchange format, CIF (Common Interchange Format) Z.106 has been standardised.

For more information on new features in SDL-2000 and information on upcoming events related to SDL, see also SDL-Forum Society.