This book discusses Aristotle’s biological views about character and the importance of what he calls “natural character traits” for the development of moral virtue as presented in his ethical treatises. It provides a new, comprehensive account of the physiological underpinnings of moral development and thereby shows, first, that Aristotle’s ethical theories do not exhaust his views about character, as has traditionally been assumed, and, second, that his treatment of natural character in the biological treatises provides the conceptual and ideological foundation for his views about habituation as developed in his ethics. This manuscript thus takes seriously Aristotle’s claim—often ignored—that nature is one of the factors through which men become “good and capable of fine deeds.” Part I (“The Physiology and Science of Natural Character”) analyzes, in three chapters, Aristotle’s notion of natural character as it is developed in the biological treatises and its role in moral development, especially as it affects women and certain “barbarians”—groups who are typically left out of accounts of Aristotle’s ethics. I also discuss its relevance for our understanding of physiognomical ideas in Aristotle. Part II (“The Physiology of Moral Development”) explores the psychophysical changes in body and soul that one is required to undergo in the process of acquiring moral virtues. It includes a discussion of Aristotle’s eugenic views, his identification of habituation as a form of human perfection, and his claims about the moral deficiencies of women that link them to his beliefs about their biological imperfections.