The astacin family of metalloendopeptidases was recognized as a novel family of proteases in the 1990s. The crayfish enzyme astacin was the first characterized and is one of the smallest members of the family. More than 20 members of the family have now been identified. They have been detected in species ranging from hydra to humans, in mature and in developmental systems. Proposed functions of these proteases include activation of growth factors, degradation of polypeptides, and processing of extracellular proteins. Astacin family proteases are synthesized with NH2-terminal signal and proenzyme sequences, and many (such as meprins, BMP-1, tolloid) contain multiple domains COOH-terminal to the protease domain. They are either secreted from cells or are plasma membrane-associated enzymes. They have some distinguishing features in addition to the signature sequence in the protease domain: HEXXHXXGFXHEXXRXDR. They have a unique type of zinc binding, with pentacoordination, and a protease domain tertiary structure that contains common attributes with serralysins, matrix metalloendopeptidases, and snake venom proteases; they cleave peptide bonds in polypeptides such as insulin B chain and bradykinin and in proteins such as casein and gelatin; and they have arylamidase activity. Meprins are unique proteases in the astacin family, and indeed in the animal kingdom, in their oligomeric structure; they are dimers of disulfide-linked dimers and are highly glycosylated, type I integral membrane proteins that have many attributes of receptors or integrins with adhesion, epidermal growth factor-like, and transmembrane domains. The alpha and beta subunits are differentially expressed and processed to yield latent and active proteases as well as membrane-associated and secreted forms. Meprins represent excellent models of hetero- and homo-oligomeric enzymes that are regulated at the transcriptional and posttranslational levels.