Meat tenderness and factors influencing it have been investigated for a long time. There are several theories explaining the process of meat tenderization. The calpain theory of tenderization has been accepted as the most feasible one. Calpains meet the basic criteria defined for factors that impact meat tenderization. These enzymes are present in a muscle cell, and in vitro experiments, when calpains are used, there are detected the same protein degradation products as in the meat after its raping. Calpains have access to myofibrils in cells. The activeness of calpains and the ratio between it and the calpastatin are correlated with the meat hardness. The higher the hardness of meat and the higher the ratio: calpain to calpastatin, the more tender meat can be obtained. Conditions under which these enzymes are active are close to conditions in the meat tissue after the death of animal. The calpain activity depends both on the survival, including genetic, and the post-mortem factors. Although the mechanism of calpain activity has not been satisfactorily explained so far, but on the basis of results obtained, it can be concluded that they play an important role in the meat tenderization process. In this paper, calpains are characterized and some factors influencing their activeness are described.