This is a review of one of the most rapidly growing fields of astronomy: gravitational lensing. The book covers not only the specific problems related to finding and understanding lensing and lensing phenomena, but also the applications of gravitational lenses in diverse areas of astronomy and astrophysics. These applications range from the structure of the Milky Way galaxy and its stellar luminosity function, to the structure of elliptical galaxies, clusters, and the size, structure and age of the universe. Contributions include reviews of MACHO searches, lensed radio sources and quasars, microlensing in distant galaxies, constraints on the mass distribution in galaxies, limits on cosmological models, measurements of the Hubble constant, and weak lensing and the masses of clusters. The volume provides a comprehensive synopsis of the state of the art in lens research as well as a summary of lens data, and it is suitable for graduate seminars, introductions to the field of gravitational lensing, and as a reference work for lens researchers.