This paper summarizes research leading to the development of a comprehensive constitutive modeling approach for granular bases, subbases, and subgrade soils. The model that was developed is based on the flow theory of plasticity. It rationally considers both the resilient and permanent deformation response, and accounts for the effects of repeated loading and saturation and drainage conditions. The model can be used to predict pavement performance, to select pavement layer thicknesses to resist permanent deformations, and to select material properties to assure adequate performance. This paper describes the conceptual model development. It also presents results from a laboratory evaluation of the constitutive model using three materials covering the range of materials typically classified as granular in pavement construction. Finally, the modeling concepts are applied to practical pavement design problems.