The aim of the present book is to fill the gap between the two, a gap which is at present filled mainly by short monographs or by the odd chapter in textbooks on modern physics. The author has found that students are interested in a more comprehensive discussion of the theory of relativity than can be given in a short monograph. The book is written at a level which the average honours physics or pass degree student should be able to understand in his second or third year at the University, and it is written primarily with these in mind, though it is hoped that it will prove useful for mathematicians who are interested in the theory of relativity as something deeper than a mathematical exercise. The book should also be suitable for electrical engineers, who seem to be showing an increasing interest in the theory of relativity, particularly in relation to electromagnetism.