The execution semantics of Alf (Action Language for Foundational UML) are defined by mapping the concrete syntax of Alf to the abstract syntax of fUML3. Alf provides a textual surface representation for a subset of UML model elements, and its main benefit is an execution semantics that allows for the generation of executable code out of Alf specifications4. This mapping ensures that Alf can be used to specify behavior in a way that is directly executable in the context of an fUML model.