The theory of ideas has represented an important object of analysis for many generations of scholars, from a philosophical, historical, philological and scientific point of view. But a simple reading of the platonic bibliography of the last fifty years shows how the interest for this fundamental theoretical question in Plato's thought was mainly addressed to some particular points of his writings, sometimes not considering the conceptual and philosophical background in which we need to understand his doctrine. That's why this book examines the notions of eidos and idea in Plato's philosophy and in the long debate his doctrines have raised, from the Ancient Academy and Aristotle to the later Middle- and Neo-platonists.