The decomposition of large systems into parts is a general principle of software design. Even more, in the scope of distributed systems a partition of the whole system into distributable components is necessary. Decisions about what constitutes a component of a system are usually either based on the behavior or on the structure of the system. Nevertheless there is a strong mutual influence between both kinds of decomposition. Despite the importance of structural and behavioral decomposition, many modeling notations and languages define the semantics of these concepts rather vague, and this may lead to incorrect implementation.This paper presents the new structuring mechanisms in the object oriented specification language SDL as of November 1999 (SDL-2000).SDL has had its primary force in structural decomposition. Object-oriented concepts as classes (types in SDL) and inheritance were introduced into the language in 1992.It has been demonstrated in the paper that it is possible for a language to provide (with formal semantics) combined mechanisms like objects with both state machines and internal structure of connected objects, composite states with well-defined interfaces, mechanisms for inheritance of such combined state machine and internal object structure. Moreover, the language also provides mechanisms for the dynamic creation of such composite structures, and mechanisms for the communication and data sharing between the contained elements. Combining the new concept of agent with the concept of interfaces makes SDL well suited for the design and development of applications in the context of distributed systems and middleware platforms (e.g. CORBA) The paper also gives a critical evaluation of these concepts and a comparison with similar approaches in UML and ROOM.