The aim of this book is to become a major reference text for gravitational-wave physics, covering in detail both the experimental and the theoretical aspects. It is the only existing book on gravitational waves to date, and it will likely remain unique for its broadness and scope. It brings the reader to the forefront of present-day research, both theoretical and experimental, assuming no previous knowledge of gravitational-wave physics. Part I of Volume 1 is devoted to the theory of gravitational waves. Here we have re-derived - in a coherent way - most of the results that we present, clarifying or streamlining existing derivations. Part II of Volume 1 is devoted to a description of experimental gravitational-wave physics. We discuss in great detail existing and planned experiments, as well as data analysis techniques.