In November 1999, the current version of SDL (Specification and Description Language), commonly referred to as SDL-2000, has passed ITU-T, an international standardization body for telecommunication. The importance and acceptance of SDL in the telecommunication industry surpasses that of UML, which can be seen as the major competitor. A crucial difference between SDL and UML is the existence of a formal SDL semantics as part of the international standard, which has a positive impact on the quality of the entire language definition. In this paper, we treat fundamental questions concerning practicability, adequacy and maintainability of the formalization approach, provide insights into the formal semantics definition and point out several eff...