The theory of relativity is tackled directly in this book, dispensing with the need to establish the insufficiency of Newtonian mechanics. This book takes advantage from the start of the geometrical nature of the relativity theory. The reader is assumed to be familiar with vector calculus in ordinary three-dimensional Euclidean space. The book is based on course material delivered at Stockholm University over a five-year period, and is intended to be of use to undergraduate and postgraduate students in astrophysics and astronomy.