The International Center for Aggregates Research Project 502 focused on pavement layers of unbound aggregate proper representation in mechanistic pavement models. The research team developed models for resilient and permanent deformation behavior from the results of triaxial tests conducted at the Texas Transportation Institute and the University of Illinois. The studies indicate that the unbound aggregate base (UAB) material should be modeled as nonlinear and cross-anisotropic to account for stress sensitivity and the significant differences between vertical and horizontal moduli and Poissons ratios. Field validation data were collected from a full-scale pavement test study conducted at Georgia Tech. The validation of the anisotropic modeling approach was accomplished by analyzing conventional flexible pavement test sections using the GT-PAVE finite element program to predict responses to load in the UAB layer and comparing these predicted responses to the measured values. Laboratory testing of the aggregate samples was conducted at the University of Illinois, and characterization models were developed for the stress-sensitive, cross-anisotropic aggregate behavior. With nonlinear anisotropic modeling of the UAB, the resilient behavior of pavement test sections was successfully predicted for a number of response variables. In addition, the stress-sensitive, cross-anisotropic representation of the base was shown to greatly reduce the horizontal tension computed in the granular base compared with a linear isotropic representation.