Since the time of the ancient Greeks, physicians have recognized a certain relatedness between the mental states of depression and mania. In the mid-Nineteenth century, French alienists proposed a 'double' or 'circular' illness consisting of alternating depressed and manic episodes and at the beginning of the twentieth century, Emil Kraepelin introduced the term 'manic-depressive insanity.' Kraepelin's broad clinical experience resulted in compelling descriptions of the symptoms of mood disorders that have arguably never been surpassed. The Kraepelinian nosology continues to provide a touchstone for modern classification systems of the mood disorders.