Enzymes are catalysts for the chemical reactions in the metabolism of all organisms and play a key role in the regulation of metabolic steps within the cells, as drug targets, and in a wide range of biotechnological applications. With respect to reaction type, they are grouped into six classes, namely oxidoreductases, transferases, hydrolases, lyases, and ligases. EC-Numbers are assigned by the IUBMB. Enzyme functional databases cover a wide range of properties and functions, such as occurrence, kinetics of enzyme-catalyzed reactions, structure, or metabolic function. BRENDA stores a large variety of different data for all classified enzymes whereas KEGG, MEROPS, MetaCyc, REBASE, CAzy, ESTHER, PeroxiBase, and KinBase specialize in either certain aspects of enzyme function or specific enzyme classes, organisms, or metabolic pathways. Databases covering enzyme nomenclature are ExplorEnz, SIB-ENZYME, and IntEnz.