This book presents the theory of relativity for students of physics and mathematics who have had no previous introduction to the subject and whose mathematical training does not go beyond the fields which are necessary for studying classical theoretical physics. Systematic and logically complete, the book develops the basic ideas of the theory of relativity in three interconnected parts. Part I, "The Special Theory of Relativity," deals with such topics as frames of reference, the Lorentz transformation, relativistic mechanics of mass points, relativistic electrodynamics, the mechanics of continuous matter, and applications. Part II, "The General Theory of Relativity," covers the principle of equivalence, the Riemann-Christoffel curvature tensor, the field equations of the general theory and their rigorous solutions, the experimental tests of the general theory, and the equations of motion of the general theory. Part III, "Unified Field Theories," presents Weyl's gauge-invariant geometry, Kaluza's five dimensional theory and the projective field theories, and a generalization of Kaluza's theory. A new Preface and two new Appendixes have been added to the Dover edition. Peter Gabriel Bergmann designed the book for classroom or individual study, and the only prerequisites are a familiarity with calculus and some knowledge of differential equations, classical mechanics, and electrodynamics. As Albert Einstein says in his Foreword to the book: "The student who makes a thorough study of the book will master the mathematical methods and physical aspects of the theory of relativity and will be in a position to interpret for himself its implications."