This revised Recommendation defines the Specification and Description Language as redefined in the ITU T 2005-2008 period known as SDL 2008. The Specification and Description Language has concepts for behavior, data description and (particularly for larger systems) structuring. The basis of behavior description is extended finite state machines communicating by messages. Data description is based on data types for values and objects. The basis for structuring is hierarchical decomposition and type hierarchies. A distinctive feature is the graphical representation. SDL 2008 is intended to be backwards compatible with uses of the SDL-92 (amended in 1996) and SDL-2000 versions for unambiguous specification and description of telecommunication systems, so that existing models can be used as SDL-2008 models. However, some features of previous versions of the language that are not used (or have been little used) and have not been supported by tools are deleted from SDL-2008. The study in 2005-2007 determines the exact feature set for SDL-2008. A criterion for a retained or new feature is to be useful for a reasonable body telecommunications system engineering applications, either for modelling or programming (being theoretically interesting or elegant is not sufficient reason for retention or addition). The presentation of the Specification and Description Language is to be revised, so that there is a clearly identified subset of the language (which all tools should support). Features such as inessential short hand syntax, annotation and repetition that have no impact on the semantics and models, and less frequently used features are separated from the core part of the language. It is being considered that Z.100 (2002) is split into a number of different Recommendations re-using the numbers Z.101, Z.102 and Z.103 not in use. Such restructuring and general revision impacts on the content of Z.104, Z.105 and Z.107. There is no current intention to use the number Z.108. The revised language feature set takes in account UML compatibility, usage, legacy and language size. The data features take in account SDL-2000, UML, SDL-RT, MSC experience, use of legacy SDL and the possible use of an embedded programming language. The compliance text takes into account the concept of a subset. The revision takes into account the proposed Z.111 and Z.119. Changes to Z.100 published only in the Implementers' guidelines for the Specification and Description Language are consolidated into the replacement(s) for Z.100.