In this article, the part of hereditary factors in the onset of depressive diseases is analysed. Primary depressive diseases may be now separated in two groups: one includes bipolar depressive syndromes. (e.i. of manic depressive type) and the other the monopolar depressive syndromes (e.i. of depressive type). A short review of twin studies and family studies in the manic depressive type bipolar depressive illnesses is made. From these studies, a discussion about genetical hypothesis concerning the manic depressive illnesses is proposed. Different genetical hypotheses are revisited, and that of a dominant X-linked disease is observed. Genetical factors are also revisited into the heterogeneous group of monopolar depressive diseases, emphasizing the different depressive subgroups of this large category. Finally, the authors also consider the possible relationship between bipolar manic depressive syndromes and schizophrenia, trying to classify the dysthymic schizophrenia.