The first protein of a group of proteins now identified as belonging to the calpain system was purified in 1976. The calpain system presently is known to be constituted of three well-characterized proteins; several lesser studied proteins that have been isolated from invertebrates; and 10 mRNAs, two each in Drosophila and C. elegans and six in vertebrates, that encode proteins, which, based on sequence homology, belong to the calpain family. The three well-characterized proteins in the calpain family include two Ca2+-dependent proteolytic enzymes, µ-calpain and m-calpain, and a protein, calpastatin, that has no known activity other than to inhibit the two calpains. A substantial amount of experimental evidence accumulated during the past 25 yr has shown that the calpain system has an important role both in rate of skeletal muscle growth and in rate and extent of postmortem tenderization. Calpastatin seems to be the variable component of the calpain system, and skeletal muscle calpastatin activity is highly related to rate of muscle protein turnover and rate of postmortem tenderization. The current paradigm is that high calpastatin activity: 1) decreases rate of muscle protein turnover and hence is associated with an increased rate of skeletal muscle growth; and 2) decreases calpain activity in postmortem muscle and hence is associated with a lower rate of postmortem tenderization. This article summarizes some of the known properties of the calpain system and discusses the potential importance of the calpain system to animal science. Key words: Calpain, calpastatin, postmortem tenderization, skeletal muscle growth