This dissertation is a study in the nature of human persons. It explores and defends materialism, the thesis that we are wholly material beings. Materialism is widely held, but its status as orthodoxy is not well-founded. Materialists rarely argue for their position, sometimes contenting themselves with mockery of rival views; and they often don't take seriously the increasingly sophisticated objections of anti-materialists. The essays comprising this dissertation remedy this unfortunate situation. In them, I develop a novel case for materialism and answer some influential objections to the view. I begin by distinguishing three questions about the metaphysics of human nature and show how their answers fit into the broader project of ontological inquiry. Then, I advance two novel arguments for my preferred version of materialism---animalism, according to which we are animals or organisms. One argument exploits crowding problems that plague non-animalist views. Another shows that animalism best accommodates the moral data. I further argue for a priority principle according to which each of us thinks our thoughts in the primary and non-derivative sense and show that a number of non-animalist views are inconsistent with this principle. These considerations constitute a cumulative prima facie case both for materialism about human persons, and for a species of that doctrine, animalism. There are powerful objections to materialism. I spend the remainder of the essays developing and answering a few of these challenges. I first examine a class of objections to materialism that stem from reflection on some peculiar cases. I show that proponents of these objections have made a mistake in ignoring questions about composition (questions about when some things make up another). Finally, I argue that cases of supervenience failure do not tell against materialism. I articulate a version of materialism that is both thoroughly materialist and compatible with such cases. I show that this position is both tenable and stable.