This article surveys evolving and competing medico-legal concepts of pyromania and insane arson. Exploiting evidence from medical jurisprudence, medico-legal publications, medical lexicography and case histories, it seeks to explicate the key positions in contemporary professional debates concerning arson and mental derangement. A major focus is the application of the doctrines of moral and partial insanity, monomania, instinctive insanity and irresistible impulse to understandings of pyromania and insane arson. The limited extent to which mental defect provided a satisfactory diagnosis and exculpatory plea for morbid arson is also explored. Additionally, this article compares and contrasts contemporary debates about other special manias, especially kleptomania. Part 2 will be published in the next issue, History of Psychiatry 21 (4).