English Summary: Book Lambda of Aristotle’s Metaphysics, commonly described as his theological treatise, is exactly what it claims to be: a general enquiry on the principles and causes of substances. Such a project implies the shaping of a causal theory on a great scale, which articulates various levels of reality, both sensitive and immobile. It also implies the act of conforming that causal theory to a threefold requirement containing most developments that will occur during the book: to warrant the primacy of principles; to ensure their efficiency; and to determine modes of unity according to the order and intelligibility of the world and to the irreducible variety of beings. Book Lambda thus appears as a fully metaphysical treatise, achieving the archeological part that Aristotle always attached to the metaphysical science, and answering many expectations of that science, whether it be construed as wisdom, the science of being as being, or the science of the substance. The present volume introduces the issues and progress of Aristotle’s metaphysical project. French Description: Communement decrit comme le traite de theologie d'Aristote, le livre Lambda de la Metaphysique est exactement ce qu'il declare etre: une enquete generale sur les principes et les causes des substances. Pareil projet implique d'elaborer dans un meme mouvement une theorie causale de grande ampleur, qui articule les differents niveaux du reel, sensible et immobile. Cela implique aussi de conformer cette theorie causale a une triple exigence, a laquelle peut se ramener l'ensemble des developpements du livre: garantir la primaute des principes, s'assurer de leur efficace, en determiner des modes d'unite respectueux de l'ordre et de l'intelligibilite du monde, comme de la diversite irreductible des etres. Le livre Lambda se presente ainsi comme un traite pleinement metaphysique, qui accomplit le volet archeologique qu'Aristote articule toujours a la science metaphysique et qui repond a bien des attentes de cette science, qu'on la designe comme sagesse, science de l'etre en tant qu'etre ou science de la substance. C'est aux enjeux et a la progression du projet metaphysique defendu par Aristote en Lambda que ce volume souhaite introduire.