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