Rhea is an expert-curated knowledgebase of chemical and transport reactions of biological interest and the standard for enzyme and transporter annotation in UniProtKB. Rhea uses the chemical dictionary ChEBI to describe reaction participants and their transformations. Rhea reactions are extensively curated with links to source literature and are mapped to other resources such as SwissLipids, MetaNetX, GO, Reactome, BioCyc and KEGG.