This thesis presents a theoretical analysis of the behavior of glasses under external perturbations, i.e. compression and shear straining. Written in a pedagogical style, it explains every facet of the problem in detail, including many crucial steps that cannot be found in the existing literature-making it particularly useful for students and as an introduction to the subject of glassy physics. In glassy systems the behavior under external compression and shear-strain is quite peculiar. Many complex phenomena are observed and grasping them fully would be a major step toward a complete theory of the glass transition. This thesis makes important advances in this direction, analyzing the behavior of glassy states in painstaking detail and reproducing it in the framework of a recently developed mean field theory for glasses that has proven extremely successful for jamming, demonstrating its predictive power in the context of metastable glassy states obtained through nonequilibrium protocols.