The study offers a new account of one of the central topics of Leibnizs philosophy: substance. It brings to light the metaphysical foundations of Leibnizs notion of substance and shows in opposition to many leading commentators that his treatment of it is governed by clear standards of significance rooted in his broader metaphysical position. Starting from Leibnizs general theory of definition founded on his views concerning the science of metaphysics the author identifies the set of basic defining features of substance for Leibniz and then provides a detailed analysis and interpretation of these features. A foundational role is given to a group of texts from the 1680s in which Leibniz provides conceptual analyses of the most general items of reality.