When doing metaphysics, it is frequently convenient and sometimes essential to rely upon various notions of fundamentality when articulating the problems, positions, and arguments at issue. But what it is, exactly, the relevant notions are supposed to track remains obscure. The goal of this dissertation is to develop and defend a theory about the metaphysics of fundamentality; by doing so, I clarify and vindicate the roles that notions of fundamentality play in metaphysics. At the theory’s core are two notions particularly prevalent in metaphysical inquiry: something’s being ‘derived’from or holding ‘wholly in virtue of’something else (the notion of grounding), and something’s being ‘reducible’to or ‘non-circularly definable’in terms of something else (the notion of reductive analysis). I begin by arguing that the notion of grounding ought to serve as a foundation for understanding the metaphysics of fundamentality more generally (“Grounding the Metaphysics of Fundamentality”). I then propose a novel account of reductive analysis that retains the traditional insight that a reductive analysis purports to describe the constituents of a property or fact and depict how they are structured together (“On What Consists in What”). I then argue that even though the two notions are distinct, facts about grounding can be understood wholly and without circularity in terms of a certain pattern of facts about reductive analysis (“Getting Grounded”). I conclude the dissertation by challenging a widespread assumption in both the literature on the metaphysics of fundamentality and in first-order disputes about what grounds what, according to which a fact is necessitated by the facts that ground it (“Against Grounding Necessitarianism”).