This book is the second edition of the well-received textbook on the theory of general relativity by Øyvind G. Grøn and Sigbjørn Hervik. It introduces the general theory of relativity and includes applications to cosmology. The reader is given an updated introduction to general relativity including the most recent developments in cosmology. The book contains a thorough introduction to tensor calculus and curved manifolds. After the necessary mathematical tools are introduced, the book provides a thorough presentation of the theory of relativity including a chapter on gravitational waves. This revised edition also contains detailed solutions to all the exercises in the first thirteen chapters for fundamental material, up to the remaining chapters devoted to advanced topics. The most important additions in the second edition are included there. The section on anisotropic cosmological models is expanded and updated, presenting an anisotropic generalization of our present standard model of the universe—the ΛCDM universe model. In addition an introduction to Noether’s theorem within classical mechanics is added in Chapter 1.