act notation, so separating the abstract syntax and semantics of the language constructs from their different concrete notations. Although the foundation constituents of the MDE approach are still evolving, some implementations of the MDE principles can be found in environments like the OMG MDA (Model Driven Architecture) [6], Model-integrated Computing (MIC) [7], Software Factories and their Microsoft DSL Tools [1], etc. We believe that the MDE paradigm can gain rigour and preciseness, necessary expecially for model analysys purposes, from the integration with formal approaches. We, in particular, try to address here the problem of defining the operational semantics of metamodel-based languages in the MDE approach. Currently, metamodelling frameworks lack of a way to specify the semantics of languages. The OMG metamodelling framework, for example, provides two main standard techniques to define languages: metamodels and UML profiles. Building metamodels is a first-class extension...