Jean Pierre Falret's once celebrated but now neglected 1854 description of "circular insanity" has not been translated into English until now. This seminal essay clearly articulated for the first time the rudimentary elements of our present diagnosis of bipolar affective disorder. It contains lucid descriptions of manic excitement and depression and the "switch" from one to the other; moreover, it emphasizes the importance of course and prognosis, as well as hereditary and epidemiologic factors. Although American psychiatry instinctively looks to the German literature for its foundations in Kraepelin, Bleuler, and Freud, the translation of Falret's essay represents an effort to trace contemporary psychiatric concepts to their origins in nineteenth-century France.