This paper describes an action semantics for UML based on\ud the Meta-Modelling Language (MML) - a precise meta-modelling language\ud designed for developing families of UML languages. Actions are de¯ned\ud as computational procedures with side-e®ects. The action semantics are\ud described in the MML style, with model, instance and semantic packages.\ud Di®erent actions are described as specializations of the basic action in their\ud own package. The aim is to show that by using a Catalysis like package\ud extension mechanism, with precise mappings to a simple semantic domain, a\ud well-structured and extensible model for an action language can be obtained.