This chapter provides an in-depth review of the biochemical changes involved in the conversion of muscle into edible meat and fish. Major advances in our understanding of the nature and structure of muscle over the past two decades have been incorporated into this chapter. The latest information on the mechanisms of postmortem tenderization is included, focusing on the involvement of lysosomal cathepsin, calpains, and proteasomes. The chapter concludes with a comprehensive discussion of meat and fish pigments, including a detailed review of the structure of myoglobins and effective methods of preservation.