The distinctive features of Plato’s philosophical system are examined, especially his rejection of naturalistic philosophic and scientific approaches and his postulation of an absolutely simple first principle. The Platonists after Plato are then considered, including Aristotle, Alcinous, Numenius, Plotinus, Iamblichus, Proclus. Platonism is conceived as a collaborative project that developed over against opposition – such as Stoicism and, in late antiquity, Christianity – as well through internal debates among its rival schools. The chapter concludes with a discussion of Platonism’s encounter with Christianity after the Edict of Milan.