Abstract Physical activity as prevention and treatment of depression Background: Almost 50 % of the population will get a mental disorder during their lifetime; the most common form is the mood disorder depression. Over the last decades, studies have shown increased prevalence rates of depression in the community. The standard forms of therapy are medication and various forms of psychotherapy. The cost of treating disease is escalating, and the health care system will never be able to meet the need for treatment in this large group of patients. Hence, development of effective self-help strategies is important. Media and other branches have already taken financial advantage of this knowledge, but is there any conclusive documented positive effect and how is it eventually modulated? Research has taken interest in the psychological effects of physical activity and many results have suggested a positive effect. The aim of this paper is to critically review a selection of the currently available literature upon the subject “How physical activity effects depression?” I have aimed to answer the following questions: 1. How does physical activity affect depression? a) Which psychological effect does physical activity promote? b) Do physical activities prevent depression? c) Can physical activity be used as an alternative to traditional treatment of depression? d) What are the consequences of using physical activity for prevention and treatment of depression for the society? 2. How is the potential therapeutic activity of physical activity modulated? Methods: I have searched Pubmed to find scientific based literature on the effect of physical activity as treatment for mental disorders. After reading headings of the 581 articles, I reviewed abstracts of 65 articles and selected 19 relevant articles for a critical literature study. Reference lists from these 19 articles where also used. In addition, general textbooks of exercise physiology and psychiatry and medical interactive databases were used. Results: There are many published articles that postulate an antidepressive effect of physical activity. Regular physical activity promotes physiological and psychological wellbeing, better self-feeling and better quality of sleep in the general population and depressed. Further, physical activity might reduce anxious, muscletention, anger and tiredness. The antidepressive effect is best documented for mild to moderate forms of unipolar depression. Compared to traditional forms of therapy, physical activity seems to be an effective and inexpensive ways to prevent and to treat mild to moderate forms of depression. It may be used as a potentially powerful adjunct to existing treatments, and a controversial scientists suggesting that it might even be used as an alternative. Physical inactivity seemed to be associated with a greater risk of developing depression, but results were not conclusive whether physical activity can prevent depression. Conclusion: A beneficial effect of physical activity on mental health is well documented, but scientific evidence of the use of physical activity as an alternative to traditional treatment is still controversial. There is also postulated a number of hypotheses of how this potential therapeutic effect is modulated, but none of them seems to be evidence based. The frequency of depression in the community is increasing and so is the need of resources for treatments. Physical activity can be a simple and inexpensive prevention and treatment for depression. This is relevant not only for doctors for the purpose of treatment, but also for health planners and policy-makers for making provisions for mental health care in the community. Although, the evidence for positive effects of physical activity on depression is growing, the clinical use is still at the beginning. Further studies on the clinical effects of exercise, details on the optimal type, intensity, frequency and duration may support the clinical use of physical activity as treatment of depression.