This revised and enhanced new edition of a well-established textbook provides a balanced overview of various areas of theoretical physics based on the use of variational principles. As well as field theory, the book deals with motion in curved spaces, the cradle of general relativity, and gravitational optics. New chapters on the relation of classical mechanics and geometrical optics as well as gravitational waves, which are considered as a true confirmation of general relativity, have been included. Each chapter has been carefully revised and enlarged. Finally, the text describes Feynman's formulation of quantum mechanics by path integrals, which gives the link between quantum and classical mechanics.The book provides a set of exercises, problems, and solutions.