This article gives an overview on OntoCAPE, a formal ontology for the domain of computer-aided process engineering. An ontology is an explicit specification of a conceptualization (Gruber, 1993), typically involving classes, their relations, and axioms for clarifying the intended semantics. A formal ontology, in particular, captures the consensual knowledge of an application domain in such a way that it can be reused and shared across software applications (Gomez-Perez et al., 2004). OntoCAPE formally represents domain knowledge about CAPE, with the intention of supporting the construction of software tools for different tasks such as knowledge management, mathematical modeling, or plant design. We argue that a useful ontology must simultaneously strive for usability and reusability and explain how these goals are achieved by OntoCAPE. (C) 2009 Elsevier Ltd. All rights reserved.