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Abstract. This chapter develops an ontology of computer-aided design, based on the function-behaviour-structure (FBS) ontology. It proposes two complementary views of the process of design. The object-centred view applies the FBS ontology to the artefact being designed. Integrating an ontology of three “design worlds”, this view establishes a framework of designing as a set of transformations between the function, behaviour and structure of the design object, driven by interactions between the three design worlds. Building on this framework, the process-centred view applies the FBS ontology to the activities defined by the object-centred view. This increases the level of detail and provides a more well-defined set of representations of these activities. Our ontological framework can be used to provide a better understanding of the functionalities required of existing and future computer-aided design support. 1