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*
specifications 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 verification.

Following the need for structuring capabilities in specification
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 state-oriented
formalisms.

In such systems, transitions regarded as atomic on the corresponding
level are allowed to be specified by computations performed by
sub-Evolving-Algebras instead of single rules.
The subsystems provide a natural way of encapsulating data and
behaviour while a computation is running.
Communication is done via distinguished locations accessible to the
participating systems.