This paper presents an approach for specifying complex, structured systems with Evolving Algebras by means of aggregation and composition. Evolving algebras provide a formal method for executable speci cations which has been employed for specifying several algorithms and programming languages. With its transition system-like rule-based syntax, the concept is as well very intuitive as well-suited for formal reasoning and veri cation. Following the need for structuring capabilities in speci cation frameworks, the paper proposes a concept for hierarchically structuring Evolving Algebras corresponding to the semantics of the system to be modeled, allowing to build up complex systems from simpler ones by several combinators. The concept can be generalized to arbitrary rule-based stateoriented formalisms. In such systems, transitions regarded as atomic on the corresponding level are allowed to be speci ed by computations performed by subEvolving-Algebras instead of single rules. The subsyste...