This paper introduces a formal theory for describing actions in terminological knowledge bases, closely related to description logics. It deals in particular with the problem of adapting the subsumption/specialization relations and the definition of inheritance from the well-formulated notions for static object concepts to dynamic action concepts. The description of action concepts integrates a formal notation of preconditions and effects similar to STRIPS planning systems. The approach suggested here anchors action descriptions in the object-concept part of the taxonomy. Object-concepts, their attributes, and relations are integrated as parameters in action descriptions, and are used in precondition and effect formulae, which specify changes in the object-concept part of the taxonomy. The definition of action concepts and their extensional semantics is based on the view of actions as transformers between world states, where preconditions and effects describe constraints on world states. This view allows a definition of inheritance and the subsumption/specialization relation for action concepts in parallel to the respective definitions for object concepts.