The Lysis is one of Plato’s most engaging but also puzzling dialogues; it has often been regarded, in the modern period, as a philosophical failure. The full philosophical and literary exploration of the dialogue illustrates how it in fact provides a systematic and coherent, if incomplete, account of a special theory about, and special explanation of, human desire and action. Furthermore, it shows how that theory and explanation are fundamental to a whole range of other Platonic dialogues and indeed to the understanding of the corpus as a whole. Part One offers an analysis of, or running commentary on, the dialogue. In Part Two Professors Penner and Rowe examine the philosophical and methodological implications of the argument uncovered by the analysis. The whole is rounded off by an epilogue on the relation between the Lysis and some other Platonic (and Aristotelian) texts.