Tenderness of meat is considered as the most important feature of meat quality. Three proteolytic systems present in a muscle were examined as those which can play a role in the postmortem proteolysis and tenderization: the system of calpains, lysosomal cathepsins and MCP (multicatalytic proteinase complex). There are several theories (enzymatic or non-enzymatic) explaining the tenderization process. The calpain theory of tenderization was recognized as the most probable. During tenderization the main structures of a cytoskeleton are degraded as well as myofibril and cytoskeletal proteins. Meat becomes soft and the process of tenderization is accompanied by changes in the ultrastructure (degradation of the Z-line and the I-band). Many studies shows that the system of calpains (especially calpain I and calpastatin) plays a major role in postmortem proteolysis and meat tenderization. However, recent studies show that proteasomes and caspases may be responsible for this process as well. This paper includes the characterization of calpains as well as describes the construction and functioning of the system of calpains. Additionally, this article presents factors influencing the activity of calpains. It was also mentioned that other systems, such as proteasomes and caspases, may be involved in postmortem tenderization of meat.