This book is a completely rewritten and substantially extended version of the previous popular book by the same author. It consists of three parts covering the major topics in relativity theory: special relativity (including relativistic optics, relativistic mechanics and electromagnetic fields in vacuum), general relativity (with sections on black holes, plane gravitational waves, de Sitter space and linearized gravity) and cosmology (including Friedman-Robertson-Walker spacetime, its dynamics and light propagation in it). The author continually stresses the main ideas and concepts, and the crucial experiments and observations. The book is based on a sound and rigorous mathematical formalism. Prerequisites include standard differential calculus, standard linear algebra and calculus of tensor fields. Every chapter ends with a set of exercises. This is a very reader-friendly book with many illustrations and diagrams and will be very useful for students as well as for research workers from other branches of the natural sciences who want to understand more about general relativity and its applications.