There is a significant risk of injury when undertaking physical activities. The risk factors for injury can be divided into extrinsic (environmental) and intrinsic (personal) factors. This article reviews the current literature on several intrinsic risk factors. Although there have been a large number of studies in this field, many are lacking in consistency of definitions and methodology. Many of the studies have been retrospective and it is often impossible to identify the baseline population. There is overwhelming evidence to suggest that female gender, age greater than 24 years, a high body mass index or a high percentage body fat (within military populations but not among civilians), low level of physical fitness at the commencement of a training programme and a past history of injury (in both populations) are risk factors for injury.