Rudolf Carnap (1891-1970) was a giant of twentieth-century philosophy. He was one of the leading figures of the logical empiricist movement associated with the Vienna Circle and one of the leaders of the analytic tradition more generally. In particular, the defining debates of this tradition involved, at its inception, Gottlob Frege, Bertrand Russell, Carnap, and Ludwig Wittgenstein, and, in a later phase, Carnap and Willard Van Orman Quine. Moreover, Carnap was engaged in significant philosophical interaction with some of the leaders of the continental tradition, including Edmund Husserl and Martin Heidegger. Finally, Carnap was a central participant in key episodes in the development of modern logic associated with Kurt Godel and Alfred Tarski; and, after emigrating to the United States, he also interacted with important American pragmatist philosophers such as Charles Morris and John Dewey. He made major contributions to philosophy of science and philosophy of logic, and, perhaps most importantly, to our understanding of the nature of philosophy as a discipline. It is impossible adequately to understand twentieth-century philosophy without appreciating Carnap's central position within it.