This study reviewed previous studies to explore the effect of season, and consequently weather, on levels of physical activity. Thirty-seven primary studies (published 1980-2006) representing a total of 291883 participants (140482 male and 152085 female) from eight different countries are described, and the effect of season on moderate levels of physical activity is considered. Upon review of the evidence, it appears that levels of physical activity vary with seasonality, and the ensuing effect of poor or extreme weather has been identified as a barrier to participation in physical activity among various populations. Therefore, previous studies that did not recognize the effect of weather and season on physical activity may, in fact, be poor representations of this behaviour. Future physical activity interventions should consider how weather promotes or hinders such behaviour. Providing indoor opportunities during the cold and wet months may foster regular physical activity behaviours year round.