Hans Reichenbach, a philosopher of science who was one of five students in Einstein's first seminar on the general theory of relativity, became Einstein's bulldog, defending the theory against criticism from philosophers, physicists, and popular commentators. This book chronicles the development of Reichenbach's reconstruction of Einstein's theory in a way that clearly sets out all of its philosophical commitments and its physical predictions as well as the battles that Reichenbach fought on its behalf, in both the academic and popular press. The essays include reviews and responses to philosophical colleagues; polemical discussions with physicists Max Born and D. C. Miller; as well as popular articles meant for the layperson. At a time when physics and philosophy were both undergoing revolutionary changes in content and method, this book is a window into the development of scientific philosophy and the role of the philosopher.