It is commonly held that our thoughts, beliefs, desires and feelings - the mental phenomena that we instantiate - are constituted by states and processes that occur inside our head. The view known as externalism, however, denies that mental phenomena are internal in this sense. The mind is not purely in the head. Mental phenomena are hybrid entities that straddle both internal state and processes and things occurring in the outside world. The development of externalist conceptions of the mind is one of the most controversial, and arguably one of the most important, developments in the philosophy of mind in the second half of the twentieth century. Yet, despite its significance most recent work on externalism has been highly technical, clouding its basic ideas and principles. Moreover, very little work has been done to locate externalism within philosophical developments in both analytic and continental traditions. In this book, Mark Rowlands aims to remedy both these problems and present for the reader a clear and accessible introduction to the subject grounded in wider developments in the history of philosophy. Rowlands shows that externalism has significant and respectable historical roots that make it much more important than a specific eruption that occurred in late twentieth-century analytic philosophy.