Domain Specific Languages (DSLs) are often defined in terms of metamodels capturing the abstract syntax of the language. For a complete definition of a DSL, both syntactic and semantic aspects of the language have to be specified. Metamodeling environments support syntactic definition issues, but they do not provide any help in defining the semantics of metamodels, which is usually given in natural language. In this chapter, we present an approach to formally define the semantics of metamodel-based languages. It is based on a translational technique that hooks to the language metamodel its precise and executable semantics expressed in terms of the Abstract State Machine formal method. We also show how different techniques can be used for formal analysis of models (i.e., instance of the language metamodel). We exemplify the use of our approach on a language for Petri nets.