Semantic externalism is the view that the meanings of referring terms, and the contents of beliefs that are expressed by those terms, are not fully determined by factors internal to the speaker but are instead bound up with the environment. The debate about semantic externalism is one of the most important but difficult topics in philosophy of mind and language, and has consequences for our understanding of the role of social institutions and the physical environment in constituting language and the mind. In this long-needed book, Jesper Kallestrup provides an invaluable map of the problem. Beginning with a thorough introduction to the theories of descriptivism and referentialism and the work of Frege and Kripke, Kallestrup moves on to analyse Putnam's Twin Earth argument, Burge's arthritis argument and Davidson's Swampman argument. He also discusses how semantic externalism is at the heart of important topics such as indexical thoughts, epistemological skepticism, self-knowledge, and mental causation. Including chapter summaries, a glossary of terms, and an annotated guide to further reading, Semantic Externalism an ideal guide for students studying philosophy of language and philosophy of mind.