The UML shares with reflective architectures the idea that self-de nition of languages and systems is a key principle for building and maintaining complex systems. The UML is now de ned by a four-layer metalevel structure, enabling a flexible and extensible de nition of mod- els by metamodels, and even a self-description of the meta-metamodel (the MOF). This metalevel dimension of UML is currently restricted to structural reflection. But recently a new extension to the UML, called the Action Semantics (AS), has been proposed for standardization to the OMG. This paper explores how this proposed extension brings a behavioural reflection dimension to the UML. Indeed, we show that it is not only possible but quite e ective to use the AS for manipulating UML models (including the AS metamodel). Besides elegant conceptual achievements, such as a metacircular de nition of the AS, reflective mod- eling with the AS leverages on the UML metalevel architecture to provide the bene ts of a reflective approach, in terms of separation of concerns, within a mainstream industrial context. A complete model can now be built as an ideal model representing the core concepts in the application, to which non-functional requirements are integrated as fully traceable transformations over this ideal model. For example, this approach paves the way for powerful UML-de ned semantics-based model transforma- tions such as refactoring, aspect weaving, application of design patterns or round-trip engineering.