The usage of UML in specific contexts (like real-time systems or process modelling) is specially appealing since it provides a standard modelling notation widely used by the software engineering community. However, suchusage usually requires to tailor (extend) the UML metamodel. The standard extension mechanisms, although enhanced in UML v.1.4, still suffer from several expressiveness limitations. In this article we identify these limitations and we define a two-tiered methodology to construct standard metamodels as extensions of the UML metamodel. Specifically, we present amethodology to extend explicitly the UML metamodel (as a particular case of MOF-model instance) and another to transform an extended UML metamodel intoa UML profile. By means of these methodologies, we are able to combine the expressiveness provided by the explicit extension with the standardization coming from the use of profiles, which allows also the usage of existing tools.