An information resource for peptidases (also termed proteases, proteinases and proteolytic enzymes) and the proteins that inhibit them. The Summary page describing a given peptidase can be found using the Name, MEROPS Identifier or source Organism indexes. The Summary describes the classification and nomenclature of the peptidase and offers links to supplementary pages showing sequence identifiers, the structure if known, literature references and more. MEROPS uses an hierarchical, structure-based classification of the peptidases. Each peptidase is assigned to a Family on the basis of statistically significant similarities in amino acid sequence, and families that are thought to be homologous are grouped together in a Clan. There is a Summary page for each family and clan, and these also have indexes. Each of the Summary pages offers links to supplementary pages.