My research centers on the role of gravity in cosmology: I use gravitational lensing and classical dynamics to study the mass distribution in the Universe, from sub-galactic to galaxy-cluster scales, with the goal of elucidating the nature of dark matter and constraining cosmological parameters. Gravitational lensing is the deflection of light from distant background sources by the intervening mass distribution, like galaxies and clusters of galaxies. Lensing is by far the best tool for the purpose of mapping out the mass distribution, as it does not rely on assumptions about the physical state of the mass. I use multiple images of distant quasars and galaxies to map out in detail the mass distribution of dark matter and ordinary matter (mainly stars) in galaxies and clusters of galaxies. Classical dynamics is a powerful means of addressing the problem of formation, evolution and relaxation of individual galaxies and clusters of galaxies. I use analytical and semi-analytical techniques to gain better understanding of dark matter halos. I am also developing theoretical models to explain the properties of dark matter halos obtained in numerical computer simulations. Our models account for the spatial and velocity distribution of dark matter within galaxy halos. They are based on fundamental physical principles, and promise to connect systems governed by Newtonian gravity with those studied by other branches of physics, like statistical mechanics and thermodynamics.