The document outlines the past, present, and future of executable UML specifications. It discusses foundational UML (fUML) which defines an executable subset of UML, the Action Language for fUML (Alf) which provides a textual notation, and the Precise Semantics of UML Composite Structures (PSCS) which extends fUML for modeling composite structures. It describes open source and commercial implementations of these specifications and notes ongoing work to finalize the specifications and extend them to cover additional UML elements like state machines. The long term vision is for a suite of specifications that define the precise semantics of different aspects of UML to enable executable modeling.