Few forms of depressive disorders can be explained purely psychologically, and this is especially true for bipolar disorders. Rather, as seen directly in organically caused depressions and substance-induced manias, many affective disorders have clear biological foundations, and for numerous other symptom patterns of this kind, this is at least very likely. In a concise space, yet detailed enough to be readable, understandable, and informative, the biological as well as psychological findings and hypotheses regarding affective disorders are presented. This includes psychoanalytic, learning-theoretical, and cognitive concepts, as well as hypotheses about neurotransmitter imbalances, endocrine peculiarities, impairments in nerve growth, or the role of inflammatory processes, alongside psychological treatment methods.