The phrase ‘in virtue of’ is a mainstay of metaphysical discourse. In recent years, many philosophers have argued that we should understand this phrase, as metaphysicians use it, in terms of a concept of metaphysical dependence called ‘grounding’.This dissertation explores a range of central issues in the theory of grounding. Chapter 1 introduces the intuitive concept of grounding and discusses some compulsory questions in the theory of grounding. Chapter 2 focusses on scepticism on grounding, according to which the recent philosophical (...) interest in the topic is misguided. In chapter 3 I discuss grounding’s explanatory roles. Chapter 4 focusses on the claim that if an entity is grounded then it is an ontological free lunch. Chapter 5 discusses and rejects the claim that groundingis a relation between facts. This conclusion raises a problem: if grounding is not a relation between facts it becomes difficult to specify the connections between grounding and explanation and grounding and necessity. But not only is it desirable to specify these relations, it is essential for establishing that grounding is able to play the explanatory roles that are discussed in chapter 3. Chapter 6 responds to this problem by outlining an approach to grounding based on David Lewis’s (2003) theory of truthmaking. Against this backdrop I discuss, in chapter 7, some issues in the logic of grounding. (shrink)