Rugby League is a game of physical contact that involves low-intensity, aerobic exercise, combined with periods of intermittent, intensive anaerobic exercise. Matches consist of two halves, each of 40 minutes, separated by a 10-minute recovery period, and are contested by 2 teams of 13 players (6 forwards and 7 backs). Whilst the amount of time spent by individual players on low-intensity exercise exceeds the duration of high-intensity exercise, the nature of the high-intensity efforts (involving sprinting, lower- and upper-body impacts and high force generation) is such that the overall intensity of the game is greatly increased. Individual players have been shown to cover distances of approximately 5000 to 8000m a game, and be involved in 20 to 40 tackles. Maximum oxygen uptake (VO2max) values of around 56 ml/kg/min have been reported for rugby league players, with no differences between the values of forwards and backs. Forwards have, however, been shown to generally have higher body mass, subcutaneous fat and fat-free mass levels than backs. Backs have been found to be quicker than forwards and produce greater leg power output when related to fat-free mass. The amount of physiological data on rugby league players and the sport of rugby league is very limited, and there is considerable scope for future research in this area.