Introduction : Quality attributes of fish flesh, including food safety, organoleptic features, nutritional quality and aptitude to industrial transformation, influence consumption and acceptability of fish as food. Fish sensorial changes and texture properties are closely linked to freshness. Along with ante mortem muscle biochemistry, postmortem biochemical processes are directly linked to final quality attributes. The understanding of postmortem mechanisms is a prerequisite for an accurate control of the quality of commercialized fish by the identification of objective markers or indicators. Postmortem tenderization is one of the most important quality attributes of fish muscle. Loss of freshness is due to a complex combination of biochemical, chemical and physical processes, and is followed by muscle spoilage due to microbiological contamination. Autolytic modifications include protease action on proteins and connective tissue, and also fat hydrolysis. Tenderization is enzymatic in nature; physio-chemical conditions (pH, osmotic pressure) may modulate the proteolytic action of endogenous enzymes. Our studies have given additional information regarding the proteolytic mechanisms involved in postmortem disorganization of fish muscle by characterizing the myofibrillar components undergoing proteolysis 1-3 as well as by studying the involved proteases especially calpains 4;5 and their effects in vitro on myofibrillar proteins 6. The knowledge of the relationship between this disorganization, loss of freshness, and textural degradation should be improved in order to identify potential freshness indicators, and develop methods to improve quality. The purpose of this paper is to give a broad overview of quality changes in fish and the possible mechanisms responsible.