The aim of this review is to assess the current evidence in scientific literature that supports the use of physical activity as a fundamental tool for primary and secondary prevention and to encourage its use conscientiously. Observational studies and international guidelines have been examined to evaluate the positive effects of physical activity as primary prevention on some of the most common diseases. We have also evaluated those studies which demonstrate that the association of physical activity with drug therapy in chronic diseases results in a better prognosis. We researched the Cochrane Controlled Trials Register, MEDLINE, PubMed up to January 2009. Furthermore, we screened references in relevant reviews and clinical trials. Sixty four studies were included in the review and cited as giving consistent evidence for the utilization of physical activity to improve health. There is strong evidence that, according to international guidelines, physical activity should be adopted as a tool in the prevention and treatment of many chronic diseases.