ChEBI is a freely available database and ontology for chemical entities of biological interest. It focuses on small chemical compounds, including natural and synthetic products used in biological processes. It includes molecular entities, groups, and classes, with an ontological classification that defines relationships between entities. ChEBI is available online at http://www.ebi.ac.uk/chebi/. The database includes a wide range of chemical entities, such as biochemical compounds, pharmaceuticals, agrochemicals, and isotopes. It uses standardized terminology and identifiers, and includes 2D chemical structures represented by InChI and SMILES. ChEBI is designed as a relational database, implemented in an Oracle server, with utility applications for data loading and web-based interfaces for public and restricted access. ChEBI contains detailed information on each entity, including names, definitions, structural diagrams, IUPAC names, synonyms, and cross-references to other databases. It also includes an ontology that classifies entities into sub-ontologies such as Molecular Structure, Biological Role, Application, and Subatomic Particle. The ontology includes relationships between entities, some of which are cyclic, to accurately represent biochemical concepts. ChEBI provides programmatic access via web services and is available in various formats, including flat-file tables, SQL dumps, and OBO format. It is freely accessible and non-proprietary, with data traceable to original sources. The database is supported by the European Commission and is used by researchers worldwide. ChEBI aims to provide a 'gold standard' for annotation of molecular entities, with a controlled vocabulary, graphical representations, and defined logical relationships. It is an essential resource for chemical data in biology, offering high-quality, standardized information for researchers.ChEBI is a freely available database and ontology for chemical entities of biological interest. It focuses on small chemical compounds, including natural and synthetic products used in biological processes. It includes molecular entities, groups, and classes, with an ontological classification that defines relationships between entities. ChEBI is available online at http://www.ebi.ac.uk/chebi/. The database includes a wide range of chemical entities, such as biochemical compounds, pharmaceuticals, agrochemicals, and isotopes. It uses standardized terminology and identifiers, and includes 2D chemical structures represented by InChI and SMILES. ChEBI is designed as a relational database, implemented in an Oracle server, with utility applications for data loading and web-based interfaces for public and restricted access. ChEBI contains detailed information on each entity, including names, definitions, structural diagrams, IUPAC names, synonyms, and cross-references to other databases. It also includes an ontology that classifies entities into sub-ontologies such as Molecular Structure, Biological Role, Application, and Subatomic Particle. The ontology includes relationships between entities, some of which are cyclic, to accurately represent biochemical concepts. ChEBI provides programmatic access via web services and is available in various formats, including flat-file tables, SQL dumps, and OBO format. It is freely accessible and non-proprietary, with data traceable to original sources. The database is supported by the European Commission and is used by researchers worldwide. ChEBI aims to provide a 'gold standard' for annotation of molecular entities, with a controlled vocabulary, graphical representations, and defined logical relationships. It is an essential resource for chemical data in biology, offering high-quality, standardized information for researchers.