ChEBI (Chemical Entities of Biological Interest) is a freely available, comprehensive dictionary of molecular entities, primarily focusing on 'small' chemical compounds. It includes natural and synthetic products used in biological processes, excluding genome-encoded macromolecules. ChEBI also covers groups and classes of entities, providing an ontological classification to specify relationships between these entities. The database is designed as a relational database implemented in Oracle, with web-based interfaces for public and restricted access. It includes unique identifiers, names, definitions, structural diagrams, chemical identifiers (InChI and SMILES), formulas, ontologies, synonyms, and cross-references to other databases. ChEBI aims to provide a high-quality, standardized vocabulary for biochemical terminology and is available at http://www.ebi.ac.uk/chebi/. The project is supported by the European Commission and aims to promote a 'gold standard' of annotation for molecular entities.ChEBI (Chemical Entities of Biological Interest) is a freely available, comprehensive dictionary of molecular entities, primarily focusing on 'small' chemical compounds. It includes natural and synthetic products used in biological processes, excluding genome-encoded macromolecules. ChEBI also covers groups and classes of entities, providing an ontological classification to specify relationships between these entities. The database is designed as a relational database implemented in Oracle, with web-based interfaces for public and restricted access. It includes unique identifiers, names, definitions, structural diagrams, chemical identifiers (InChI and SMILES), formulas, ontologies, synonyms, and cross-references to other databases. ChEBI aims to provide a high-quality, standardized vocabulary for biochemical terminology and is available at http://www.ebi.ac.uk/chebi/. The project is supported by the European Commission and aims to promote a 'gold standard' of annotation for molecular entities.