This dissertation is a critique of synthetic ethical naturalism (SEN). SEN is a view in metaethics that comprises three key theses: first, there are moral properties and facts that are independent of the beliefs and attitudes of moral appraisers (moral realism); second, moral properties and facts are identical to (or constituted only by) natural properties and facts (ethical naturalism); and third, sentences used to assert identity or constitution relations between moral and natural properties are expressions of synthetic, a posteriori necessities. The last of these theses, which distinguishes SEN from other forms of ethical naturalism, is supported by a fourth: the semantic contents of the central moral predicates such as 'morally right' and 'morally good' are fixed in part by features external to the minds of speakers (moral semantic externalism). Chapter 1 introduces SEN and discusses the most common motivations for accepting it. The next three chapters discuss the influential "Moral Twin Earth" argument against moral semantic externalism. In Chapter 2, I defend this argument from the charge that the thought experiment upon which it depends is defective. In Chapters 3 and 4, I consider two attempts to amend SEN so as to render it immune to the Moral Twin Earth argument. I show that each of these proposed amendments amounts to an abandonment of SEN. Chapter Five explores Richard Boyd's proposal that moral goodness is a "homeostatic property cluster." If true, Boyd's hypothesis could be used to support several metaphysical, epistemological, and semantic claims made on behalf of SEN. I advance three arguments against this account of moral goodness. In the sixth chapter, I argue that moral facts are not needed in the best a posteriori explanations of our moral beliefs and moral sensibility. Because of this, those who accept a metaphysical naturalism ought to deny the existence of such facts or else accept skepticism about moral knowledge. In Chapter 7, I consider a counterargument on behalf of SEN to the effect that moral facts are needed in order to explain the predictive success of our best moral theories. I show that this argument fails.