This Recommendation is a part of the set of Specification and description language Recommendations for SDL-2008. It provides an overview and common material (such as conventions and tool compliance). It gives concepts for behaviour, data description and (particularly for larger systems) structuring. The basis of behaviour 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 backwards compatible with previous versions of SDL while adding significant new features. This Recommendation is revised as part of the restructuring of the ITU-T Z.100 series for SDL-2008.