A distinguished group of scholars of ancient philosophy here presents a systematic study of the twelfth book of Aristotle's Metaphysics. Lambda, which can be regarded as a self-standing treatise on substance, has been attracting particular attention in recent years, and was chosen as the focus of the fourteenth Symposium Aristotelicum, from which this volume derives. At the Symposium, each of Lambda's ten chapters was taken in turn as the subject of a session at which a specially written paper was read to and discussed by the assembled symposiasts. (The ninth chapter commanded two sessions by dint of its particular difficulty.) The papers have been revised in the light of discussion, and are now offered to a wider audience as a discursive commentary on points of particular philosophical interest covering all of Lambda. Michael Frede's extensive Introduction aims to give a broader view of Lambda as a whole and the problems it raises, and thus to provide the context for the discussion of each of the chapters. This volume will be a resource of great value and interest for anyone working on ancient metaphysics and theology.