The design and the requirements of modern computer-based systems have reached a complexity level that calls for the use of models for the verification of non functional requirements since the beginning of their design cycle. Such systems are however too complex to be modeled directly in a simple unstructured formal language like Queueing Networks or Petri Nets. SIMTHESys (Structured Infrastructure for Multiformalism modeling and Testing of Heterogeneous formalisms and Extensions for SYStems) is a novel approach to multiformalism compositional modeling, that is based on the possibility of freely specifying the dynamics of the elements of a formal modeling language in an open framework. This is obtained by the application of consolidated metamodeling foundations to the description of models, together with the concept of behavior as a bridge between formalism dynamics and solution techniques. In this paper the main concepts of the SIMTHESys approach are presented, together with a running example of how SIMTHESys copes with performance evaluation of multiformalism models.