The purpose of this paper is to review those studies which have attempted to delineate causative factors of hamstring strains. Anatomy and biomechanics of the hamstring muscle group are reviewed. Mechanisms of injury and various etiological factors are presented. Decreased hamstring muscle flexibility and decreased hamstring muscle strength were factors most frequently studied, yet the relationship of these factors to the incidence of hamstring strains remains unclear. Future studies should seek to test hamstring muscle flexibility and strength under conditions simulating the proposed mechanisms of injury. Results of further research examining decreased hamstring muscle flexibility and strength must be viewed in light of age, levels of maturity, history of previous injury, demands of specific activities, and psychological characteristics of the participants.J Orthop Sports Phys Ther 1984;5(4):184-195.