This book presents an introduction to Einstein's Theory of Relativity, which has become a necessary part of the theoretical equipment of every physicist. Even if we regard the Einstein theory of relativity merely as a convenient tool for the prediction of electromagnetic and optical phenomena, its importance to the physicist is very great, not only because its introduction greatly simplifies the deduction of many theorems which were already familiar in the older theories based on a stationary ether, but also because it leads simply and directly to correct conclusions in the case of such experiments as those of Michelson and Morley, Trouton and Noble, and Kaufman and Bucherer, which can be made to agree with the idea of a stationary ether only by the introduction of complicated and ad hoc assumptions.