There is consensus that genetic factors are the most important risk factor for both bipolar and unipolar disorders. Concerning other risk factors there seems to be complex interactions between biological, psychological and sociological factors. For bipolar disorder, suggestive findings have been provided concerning pregnancy and stressful life events and female gender and stressful life events are the most reliable factors for unipolar disorder. Clinical and research recommendations are given in order to create strategies for the prevention of affective disorders.