In this thesis, I argue that physicalism should be understood to be the view that mental properties are realized by physical properties. In doing this, I explore what the realization relation might be. Since realization is the relation that should help us formulate physicalism, I suggest that the theoretical role of realization consists in explaining some of the things that physicalists wish to explain. These are: How are mental properties metaphysically necessitated by physical properties? How are mental properties causally efficacious? (...) A theory of realization should provide resources for answering these questions. Having identified the theoretical role of realization, I discuss several theories of realization, but then focus on the subset view of realization. According to the subset view, a property P realizes a property Q if and only if the causal powers of Q are a proper subset of the causal powers of P. I argue that the realization relation as it is formulated by the subset view is a promising candidate to play the theoretical role that I want realization to play. I then investigate how this theoretical role is occupied. In doing so, I provide a general metaphysical framework that the defenders of the subset view can appeal to. This framework specifies in what ways properties are related to their causal powers. Discussing some problems that the subset view faces, I propose my own version of the subset view. I argue that a property P realizes a property Q if and only if the causal powers of Q are a proper subset of the causal powers of P, and P is more fundamental than Q. Thanks to the requirement that a realized property is less fundamental than its realizers, two things that the original version of the subset view cannot explain are guaranteed: first, fundamental properties are not realized; second, arbitrary conjunctions of properties do not realize their conjuncts. By showing how a theory of realization can help us explain some of the things that physicalists typically wish to explain, I also show that a non-reductive variety of physicalism does not face the problems that it is commonly thought to face. (shrink)