Complex systems demand diversity in the modeling mechanisms. One way to deal with a diversity of requirements is to create flexible modeling frameworks that can be adapted to cover the field of interest. The downside of this approach is a weakening of the semantics of the modeling frameworks that compromises interoperability, understandability, and analyzability of the models. An alternative approach is to embrace heterogeneity and to provide mechanisms for a diversity of models to interact. This paper reviews an approach that achieves such interaction between diverse models using an
, which is a deliberately incomplete semantics that cannot by itself define a useful modeling framework. It instead focuses on the interactions between diverse models, reducing the nature of those interactions to a minimum that achieves a well-defined composition. An example of such an abstract semantics is the
, which can handle many heterogeneous models that are built today, and some that are not common today. The actor abstract semantics and many concrete semantics have been implemented in Ptolemy II, an open-source software framework distributed under a BSD-style license.