The objective of this chapter is to first present the basic extension mechanisms proposed by UML. We then propose an extension to facilitate the modeling of specific applications. UML provides three extension mechanisms to allow the modelers to make some common extensions without having to modify the language of modeling underlying “Tag Values,” “Restrictions,” and “Stereotypes.” There are several adaptations of UML, which occasionally exceed the extension mechanisms of UML. In this chapter, we present our proposal of “Evolutionary Stereotypes.” We also present a tool that incorporates evolutionary stereotypes within two modules: the model checker and the dynamic semantics. A case study about time restrictions in a real-time system is shown. The reason for this proposal is that UML provides mechanisms for doing extensions; but, UML does not assure incorporation of new elements to the meta-model with dynamic semantics.