Model-driven Engineering (MDE) is an emerging approach for software development. It uses metamodels to define language (or formalism) abstract notation, so separating the abstract syntax and semantics of the language from their different concrete notations. However, metamodelling frameworks lack of a way to specify the semantics of languages, which is usually given in natural language. We claim that the MDE paradigm can gain rigor and preciseness from the integration with formal approaches, and we propose the integration with the ASMs to define a unified methodology for metamodel-based language syntax and semantics definitions.