Ontologies are becoming a recognizedvehicle for knowledge reuse, knowledge sharing,andmodeling. This paper presents PLANET, an ontology for representing plans. To show that PLANET is a comprehensive ontology that can be reused to build new applications, we describe several specializations of it to represent plans in three different real-world domains thatweremade by addingminor extensions to PLANET’s general definitions and classes. In pastwork, we have developed several plan evaluation systems in these domains, and have integrated them in several occasions with plan editors and plan generation systems. For each of these integrations, and often for each system in the integration, a new format for exchanging plans was devised. PLANET can represent all of the plans that we and others have used in these domains, providing a standard universal format for knowledge sharing that can be used as an interlingua in integrated planning applications. Finally, the paper discusses how we have used PLANET as a knowledge modelling tool to design representations for courses of action in a military domain, guiding us to make useful distinctions and modelling choices.