Peptidases are catalytically active proteins (enzymes) that cleave peptide bonds in proteins and peptides by hydrolysis. Not only do peptidases break down proteins and peptides so that the amino acids can be recycled and used during growth and remodelling, but they are also important for modifying proteins. These processing events ensure proteins are sent to the correct cellular or extracellular locations, are activated or inactivated when required, and that biologically important peptides are excised. There are six catalytic types (serine, cysteine, threonine, aspartic, glutamic and metallo). Peptidases can be classified by sequence similarities into approximately 250 families, and these in turn can be arranged into approximately 60 clans by comparing tertiary structures. Key Concepts: A peptidase is a catalytically active protein that cleaves one or more peptide bonds in a protein or peptide by hydrolysis. There are six different catalytic types of peptidases. Comparison of the amino acid sequences of peptidases allows them to be classified into about 250 families. Comparison of the structural folds of peptidases allows them to be classified into about 60 clans. A peptidase that acts only within three residues of the amino or carboxyl termini of a substrate protein is known as an exopeptidase. A peptidase that cleaves any bond in a substrate protein that is more than three residues from the amino or carboxyl termini is known as an endopeptidase. A peptidase has an active site containing amino acids that are important for catalysis and which are usually conserved in all active members of a family. The active site of a peptidase is flanked by substrate binding pockets, each of which accommodates one residue from the substrate. Some peptidases have additional substrate binding sites remote from the active site that are known as exosites.