An overview of the use of ontologies in process systems engineering, which has increased continuously since the early 1990s, is presented. Ontologies have formal semantics that can be exploited when part of the information is missing, enabling engineers and stakeholders to independently develop partial descriptions of the same artifact (product, plant, or process) and check consistency when the descriptions are combined. Likewise, ontologies are useful for generating new conclusions from existing data. Because of their intrinsic foundation in mathematical logic, ontologies provide the structure and semantics for validating information.