AbstractAn ontology is a domain of knowledge structured through formal rules so that it can be interpreted and used by computers. Ontologies are becoming increasingly important in bioinformatics because they can be linked to the information in databases and their knowledge then used to query the databases. Typical examples in current use are the Gene Ontology, which incorporates much of our knowledge about gene products, and ontologies of developmental anatomy, which, for example, facilitate tissue‐based queries to gene expression databases both textually and spatially. This article considers the production, formulation and types of bio‐ontologies together with the reasons why they are so useful. © BioEssays 25:501–506, 2003. © 2003 Wiley Periodicals, Inc.