In recent years there has been a shift in emphasis in livestock production away from increased muscle growth towards improved meat quality. The final eating quality of meat depends on a number of organoleptic properties including appearance, colour, fat content, taste, texture and tenderness. Whilst colour and fat content are important in influencing meat purchase, consumer studies indicate that it is the degree to which muscle tenderises after slaughter that is the most important factor contributing to overall meat quality (Warkup et al, 1995). Despite efforts to standardise breeding, husbandry, nutrition, transport, lairage and slaughter regimes, ensuring a consistently tender product still remains difficult to control or predict. The problem is international, with beefsteak toughness a major concern in the USA and pork toughness difficult to eradicate in the UK. The tenderisation process involves complex changes in muscle metabolism in the immediate post slaughter period and is dependent on genetic makeup, protein complement, metabolic status and environmental factors such as physiological stress. In the early postmortem period, glycogen depletion, lactic acid accumulation, pH decline and rate of entry and exit into rigor can all influence the ultimate tenderness of the meat some 8 - 20 days later following a period of conditioning (Goll et al, 1995). However, the main determinant of ultimate tenderness appears to be the extent of proteolysis of key target proteins within muscle fibres (Taylor et al, 1995). Research in all major livestock species has pointed to the calpain proteolytic enzyme family being a major factor responsible for key peptide bond cleavage (Koohmaraie, 1996). Whilst opinion is divided as to which isoform of calpain is the most important under specified conditions, most workers agree that the major factor is the level at slaughter of the specific calpain inhibitor calpastatin. The evidence for this is reviewed here, highlighting potential means of regulating the system in order to assure a consistently high quality tender product.