A study was commenced in 1998 at the University of New South Wales (UNSW) to investigate the dynamics of head impacts in football which resulted in concussion. Sixty-eight cases of medically verified concussion from Australian Rules Football and 32 from Rugby Union and Rugby League were analyzed. Video of each injury event was analyzed to obtain descriptive data regarding the head impact site and striking object. The video was analyzed quantitatively to obtain estimates of the closing speeds. A secondary analysis was undertaken using the conservation of momentum and energy relationships to estimate the change in velocity of the head during the impact, the change in momentum of the head, and the energy imparted to the head. Ninety-seven cases involved direct head contact, whereas three cases involved impulsive loading via the trunk. The majority of impacts were to the tempero-parietal region, and the striking body segment was commonly the arm or shoulder/thorax. The mean change in velocity of the head and head impact energy for all 97 cases of direct head impact were 4 m x s(-1) and 56 J, respectively. Head impact energy can be used as a performance criteria for testing and developing headgear for rugby and Australian rules football.