Model-Driven Engineering intensively uses models and model transformations. Transformation tools ensure that the target model conforms to the target metamodel, so that it is syntactically correct. However, there is few assistance, or none at all, to guarantee that the semantics is preserved during the transformation. This is mainly due to the absence of an explicit semantics within the models. Models bring the syntax while the related (application-specific) analysis tools bring their own semantics. We propose here a model-driven approach to describe a formal and explicit semantics as a separate model. This formal semantics can then be attached to different UML /DSL models and a UML /DSL model can be executed with different semantics.