A conducted grand tour of the fundamental theories which shape our modern understanding of the physical world. This book covers the central themes of spacetime geometry and the general-relativistic account of gravity; quantum mechanics and quantum field theory; gauge theories and the fundamental forces of nature, statistical mechanics and the theory of phase transitions. The basic structure of each theory is explained in explicit mathematical detail with emphasis on conceptual understanding rather than on the technical details of specialized applications. Straightforward accounts are given of the standard models of particle physics and cosmology, and some of the more speculative ideas of modern theoretical physics are examined. This book is unique in bringing together the diverse areas of physics which are usually treated as independent. Designed to be accessible to final year undergraduates in physics and mathematics and to provide first year graduate students with a broad introductory view of theoretical physics, it will also be of interest to scientists and engineers in other disciplines who need an account of the subject at a level intermediate between semi-popular and technical research.