Semantic externalism is the thesis that the contents of some words and thoughts depend in part on properties external to the person who entertains them. In a departure from the widely held doctrine of internalism, externalists maintain that not all mental content is local to the mind. That view, however, seems to some philosophers to be at odds with our ordinary intuitions about self-knowledge. This book shows that the debate over the compatibility of externalism and self-knowledge has led to the investigation of a variety of topics, including the a priori, transmission of epistemic warrant, question-begging reasoning, and the semantics of natural-kind terms, as well as other issues crucial to epistemology, the philosophy of mind, and the philosophy of language. The essays in the book make clear that externalism and self-knowledge raise many questions and that there are many paths to answering them. The best way to deal with the competing arguments, the editor claims, is to follow a principle of doxastic conservatism, which recommends that, when possible, one should favor the strategy that best accommodates all of the most accepted intuitions at stake.