This chapter examines one of the most prominent notions in recent metaphysical discourse: the concept of grounding. Broadly speaking, x grounds y if and only if y depends on x for its existence, nature, or truth. Interest in this relation arises, at least in part, from the idea that grounding might serve as a device (or perhaps the most effective device) for articulating a hierarchical view of reality—one in which reality is organized into layered ontological strata. Viewed through the lens of grounding, many other significant metaphysical concepts tied to this stratified conception of reality, such as fundamentality and ”degrees of reality,” can be clarified and defined. As with any relation, the grounding relation can be explored by analyzing its formal features and the types of entities it connects. A substantial portion of this chapter is dedicated to these issues. Grounding is typically characterized as an irreflexive, asymmetric relation between facts and is further distinguished by features such as non-monotonicity, well-foundedness, and hyperintensionality. The remainder of the chapter explores grounding’s connections to other explanatory metaphysical relations, including reduction, supervenience, and causation.