This collection of original papers on the special and general theories of relativity is an unabridged translation of the 4th edition of Das Relativitatsprinzip, together with a revised edition of an additional paper by H. A. Lorentz.CONTENTS: I. "Michelson's Interference Experiment" by H. A . Lorentz. II. "Electromagnetic Phenomena in a System Moving with any Velocity Less than that of Light" by H. A . Lorentz. Ill. "On the Electrodynamics of Moving Bodies" by A. Einstein. IV. "Does the Inertia of a Body Depend Upon its Energy-Content?" by A. Einstein. V. "Space and Time" by H. Minkowski. VI. "On the Influence of Gravitation on the Propagation of Light" by A. Einstein. VII. "The Foundation of the General Theory of Relativity" by A. Einstein. VIII. "Hamilton's Principle and the General Theory of Relativity" by A. Einstein. IX. "Cosmological Considerations on the General Theory of Relativity" by A. Einstein. X. "Do Gravitational Fields Play an Essential Part in the Structure of the Elementary Particles of Matter?" by A. Einstein. XI. "Gravitation and Electricity" by H. Weyl."The book constitutes an indispensable part of a library on relativity," Nature. "It is really a thrill to read again the original papers by these giants," School Science and Mathematics. "Warmly recommended," Quarterly of Applied Mathematics.