The aim of this book is to develop a metaphysical account of mechanisms. So far, the new mechanistic literature has mainly focused on epistemic issues such as scientific explanation, scientific discovery, and causal modelling. This book takes a difference stance: I will investigate in which sense mechanisms are things in the world; what our ontology has to look like in order for mechanisms to exist, and its implications for causation, levels, and part–whole relations; and how metaphysics and scientific explanation relate to each other. I will discuss whether the metaphysics of mechanisms is reductionist, and whether it leaves room for the causal efficacy of higher-level phenomena. Finally, I hope to provide a starting point for new projects on issues in the philosophy of mind, such as non-reductive physicalism as a solution to the mind–body problem.