The present document establishes a set of guidelines for the formal use of Specification and Description Language (SDL) for descriptive@ rather than detailed design@ purposes. It also provides some guidance on the use of Message Sequence Charts (MSC)@ Abstract Syntax Notation 1 (ASN.1) and the Unified Modeling Language (UML) when used in conjunction with SDL. The objective of the guidelines is to provide assistance to rapporteurs of protocol standards so that the SDL that appears in ETSI deliverables is formally expressed@ easy to read and understand and at a level of detail consistent with other standards. The present document applies to all standards that make use of SDL to specify protocols@ services or any other type of behaviour. Users of the present document are assumed to have a working knowledge of SDL and@ where necessary@ MSC@ ASN.1 and UML. It should not be considered to be a tutorial in any of these notations and should be read in conjunction with EG 201 383 [1]@ EG 201 015 [2] and EG 201 872 [3].