The UMLlac ks precise and formal foundations for several constructs such as transition guards or method bodies, for which it resorts to semantic loopholes in the form of “uninterpreted” expressions. The Action Semantics proposal aims at filling this gap by providing both a metamodel integrated into the UML metamodel, and a model of execution for these statements. As a future OMG standard, the Action Semantics eases the move to tool interoperability, and allows for executable modeling and simulation. We explore in this paper a specificity of the Action Semantics: its applicability to the UML metamodel, itself a UML model. We show how this approach paves the way for powerful metaprogramming capabilities such as refactoring, aspect weaving, application of design patterns or round-trip engineering. Furthermore, the overhead for designers is minimal, as mappings from usual objectoriented languages to the Action Semantics will be standardized. We focus on an approach for expressing manipulations on UML models with the upcoming Action Semantics. We illustrate this approach by various examples of model transformations.