One hundred years ago – in 1899 – Emil Kraepelin, Professor of Psychiatry in Heidelberg and later in Munich – created, in two very important pieces of work, the concept of "manisch-depressives Irresein" ("manic-depressive insanity"). The first was entitled Die klinische Stellung der Melancholie(The Clinical Position of Melancholia), and the second publication was the sixth edition of his textbook. In the same year Kraepelin's pupil and colleague, Wilhelm Weygandt, published his book Über die Mischzustände des Manisch- Depressiven Irreseins (On the Mixed States of Manic-Depressive Insanity). A century after Kraepelin's creation of "manic-depressive insanity", we celebrate. Is this really appropriate? We believe it is firmly established that the "folie circulaire" of Jean-Pierre Falret or the "folie à double forme" of Jules Baillarger differs from recurrent depression, which is also different from Kraepelin's "manic-depressive insanity". Yet the answer to the question of xvi Preface whether it is appropriate to celebrate is clear: Yes. This not only because the work of Emil Kraepelin is fundamental in the true sense of the word. There can be no doubt that Emil Kraepelin is the most important founder of modern psychiatry. Just one of the many reasons for this opinion is his enormous contribution to the definition, description and diagnosis of affective disorders.