Both Kant and Sidgwick recognize a deep tension between moral and prudential reasons. And, both argue, if acting morally must sometimes come at the cost of one’s own happiness, then we should be skeptical about the possibility of having a completely rationalized account of practical reason. Ancient eudaimonism might seem to offer a solution to the dualism of practical reason, and without any theological assumptions. Eudaimonism holds that there is indeed a single ultimate rational aim that can accommodate all of our ethical concerns, namely, the pursuit of an agent’s own happiness. Kant and Sidgwick, however, are pessimistic about the possibility of eudaimonist theories successfully reconciling both prudential and moral concerns under a single system of practical reason. This chapter reviews the challenges that Kant and Sidgwick pose to eudaimonism and argues that there are resources within ancient theories—and within Aristotle in particular—to potentially resolve at least some of their concerns. Much turns on how exactly we understand ancient eudaimonist theories and, the chapter argues, this question is far from settled.