Most volumes in the Cambridge Companion series deal with philosophers. Following volumes on Galileo, Newton, and Darwin, this is the fourth Companion devoted to a major scientist. The inclusion of figures such as Galileo, Newton, and Einstein in this series reminds us that natural philosophy traditionally included what we today call physics, and that up to the middle of the twentieth century a clear border between physics and philosophy did not exist. Few would dispute that Einstein was the greatest natural philosopher of the twentieth century in this traditional sense. Not only was he centrally responsible for the formulation of the two most important fundamental theories of modern physics, the theory of relativity and quantum theory, he also devoted considerable effort to explaining and defending his views on the epistemology and methodology of physics. His writings have had an enormous impact on the development of philosophy of science in the twentieth century, and beyond that on analytic philosophy more generally. Many of the philosophers relevant for the rise of analytic philosophy in the first half of the twentieth century, especially in the German-speaking countries, such as Moritz Schlick, Hans Reichenbach, Rudolf Carnap, or Karl Popper, were concerned with interpreting and developing Einstein's work in a general philosophical context. This volume is meant to provide an introduction to Einstein's work that is comprehensive and accessible to the general reader. Most of the chapters in this volume deal with Einstein's pathbreaking contributions to physics, in relativity theory, quantum theory, and statistical physics. However, there are also several chapters on Einstein's reflections on the foundations of physics (especially quantum mechanics), scientific methodology, epistemology, and politics. In the introduction to this volume, we provide a more detailed guide to its contents. Here we want to acknowledge some of the more important debts we accrued in putting together this Companion.