In this account of the pan that heat plays in the conception and development of living substances according to Aristotle, I begin by examining the concept of heat. I discover that Aristotle uses a distinction between to thermon and thermotes: the former is, in living substances, material; the latter is never material, being the powerful aspect of heat. For example, an animal possesses heat (to thermon) which maintains it through its power (thermotes) to concoct. I then turn to the biological works. Conception, it seems, does not fit the standard account of change, but is rather a concoction, performed by the heat of the semen. Nor is the usual account of conception ascribed to Aristotle adequate: I attempt to demonstrate that he held a more moderate account in which pneuma, the nature of which is to thermon, is transmitted to the embryo. I then examine the development of the embryo, which is performed using to thermon as a tool. The transmission and development of the rational psuche in particular has often caused problems: I offer an account of the transmission of psuche from parent to embryo, and describe the part that pneuma plays in this transmission and in the development and operation of the various levels of psuche. Development extends from foetal development until adulthood, and this poses another problem for the standard account of change as it appears to be neither substantial nor accidental change, yet these are apparently exhaustive possibilities. I conclude that development, like conception, is a concoction performed by the vital heat. Finally, I turn to the conception and development of spontaneously generated animals, and of abnormal animals such as monsters. I demonstrate the relationship between these generations and sexual generation, and the significance of heat and pneuma.