In the year 1899 there occurred an event which has had great consequence for psychiatry. This was the publication of the sixth edition of Emil Kraepelin's textbook, where he introduced for the first time his distinction between manic-depressive insanity and dementia praecox. It was a distinction which rapidly became accepted almost everywhere in the world, and it still forms the basis of our thinking about the nature of the functional psychoses. Kraepelin's concept of mania was quite different from the concept of mania held during most of the nineteenth century; and so, historically speaking, there are two manias, more or less sharply separated by the Kraepelinian revolution. The purpose of the present essay is to give some account of the term mania in its pre-Kraepelinian sense and of the events which led Kraepelin to his new concept; and also (in Part II) to put forward a new idea of why this revolution came about.