The study offers a new account of one of the central topics of Leibniz’s philosophy: substance. It brings to light the metaphysical foundations of Leibniz’s 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 Leibniz’s 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 1680’s in which Leibniz provides conceptual analyses of the most general items of reality.