For individuals who are just beginning to exercise, the very unfit, the elderly and persons suffering from psychiatric disorders, low intensity activity has important potential psychological benefits. Several studies indicate that mental health can be improved by low- or moderate-intensity activity. In 2 studies it could be demonstrated that aerobic exercise plus counselling was more effective in the treatment of depressive disorders than counselling alone. Cross-sectional community studies clearly reveal that after controlling for potential sociodemographic and health-related confounding variables the risk of depression is significantly higher for physically inactive individuals compared with regular exercisers. No final conclusions can be drawn from longitudinal field studies on the predictive value of physical activity on the degree of depressive symptoms. Several biological and psychological hypotheses have been proposed to explain the association between physical activity and mental health, however, there is still a lack of an integrated theoretical model.