This book contains the proceedings of the International Astronomical Union Symposium no. 225, held in July 2004 at the Ecole Polytechnique Federale de Lausanne (EPFL), in Lausanne, Switzerland. The meeting focused on the applications of gravitational lensing to cosmological physics, and this book summarizes the most recent theoretical and observational developments. Chapters are written by leading scientists in the field, and cover testing gravitation theory with gravitational lensing; weak gravitational shear and gravitational magnification of galaxies; anisotropy of the cosmological microwave background as a probe of the dark matter power spectrum and the growth rate of perturbations in the universe, and its implications for investigating the properties of dark energy. The properties of dark halos from galaxy to super-cluster of galaxies scales are presented using mass reconstruction techniques. This book is a valuable resource for professional astronomers and graduate students in astronomy, physics and astro-particle physics.