A key factor for the successful adoption of Model-Driven De-velopment (MDD) techniques is the ability to verify models, which can be achieved by model simulation and testing. This is already a reality for UML models, due mainly to the UML action language. However, this action language is purely object-oriented, which implies it needs to be extended to model applications of new software development paradigms, such as aspect-oriented applications. These extensions also require modifying the tools that execute the UML models, which, with the currently available tools, is often not pos-sible. Furthermore, at the current time, there are no tools fully compliant with the UML action language. In order to overcome this limitation, this paper presents Pópulo, an ex-tensible UML model debugger, which interprets the UML action language, compliant with the standard, and that can be customised for executing profiled (i.e. extended) UML models.