The conversion of muscle into meat is a complex process involving many biochemical and physical changes. Muscle tissue is converted from an extensible, metabolically active system into one that is inextensible and quiescent with regard to its biochemical reactions. The speed and extent of postmortem metabolism has a profound effect on the properties of the muscle and its subsequent use for food. Also the extent of shortening during the postmortem period affects the meat’s textural properties. This chapter summarizes our understanding of muscle’s structural components, its metabolic pathways, and its behavior postmortem. Several books1-5 and reviews6-13 may be consulted for more extensive coverage of these topics.