This paper describes four nosological categories of depressive disorder according to the fundamental or prime cause: endogen depression, situative depression, psychogen depression and somatogen (also pharmacogen or addictive) depression. Recent advances in neurobiology provide the commun pathogenic mechanism distribuited in neurochemical, neuroendocrine and neuroinmune factors, with at the end a cellular and molecular sequence beyond the synapse. There is an increased risk of depression multiplied by three or four in the elderly, obese, unemployed and inmigrant and it is very frequent in terminally ill patients with a pervasive desire for death. Finally, eight personalized preventive guidelines enable to decrease the individual risk of depression in more than a fifty per cent.