This book offers a fresh analysis of the account of Peripatetic ethics in Cicero's On Ends 5, which goes back to the first-century BCE philosopher Antiochus of Ascalon. Georgia Tsouni challenges previous characterisations of Antiochus' philosophical project as 'eclectic' and shows how his reconstruction of the ethics of the 'Old Academy' demonstrates a careful attempt to update the ancient heritage, and predominantly the views of Aristotle and the Peripatos, in the light of contemporary Stoic-led debates. This results in both a hermeneutically complex and a philosophically exciting reading of the old tradition. A case in point is the way Antiochus grounds the 'Old Academic' conception of the happy life in natural appropriation (oikeiosis), thus offering a naturalistic version of Aristotelian ethics.