The scientific successes of the last 400 years strongly suggest a view on which things are organized into layers, with phenomena in higher layers dependent on and determined by what goes on below. Philosophers have recently explored the idea that we can make sense of this view by appeal to a relation called grounding. In Fundamental Things, Louis de Rosset develops the rudiments of a theory of grounding and applies that theory to questions concerning the contents of the layers and the relations among them. This theory specifies what grounding is and how it relates to relevant forms of explanation. It addresses arguments for skepticism about grounding and draws points of contrast between a grounding-centered approach to relative fundamentality and other approaches. deRosset then turns to a demonstration of how the theory of grounding bears fruit in investigating questions concerning (1) how to distinguish between truths that say how objective reality is in itself, quite independently of us, and truths that do not; (2) the nature of truth; and (3) the relation between fundamental physical facts and the rich panoply of other facts that depend on and are determined by them, including facts concerning our own doings. The aim is to advance our understanding of one of the deepest and thorniest questions which the stunning scientific achievements of the last 400 years pose: how higher-level phenomena fit into an ultimately physical world.