The main objective of this paper is to present the state-of-the-art in the anisotropic characterization of unbound aggregate layers to support the development of the next generation of highway as well as airport pavement infrastructure by employing advanced technology and detailed knowledge gathered within the last one and a half decades in characterization of the actual stress- and direction- (anisotropic) dependent aggregate behavior. From advanced triaxial tests conducted in the laboratory, unbound aggregate materials are clearly shown to have greater modulus in the vertical direction, which can be adequately modeled using cross-anisotropic, stress-dependent modulus characterization models. Field validations of the anisotropic aggregate layer behavior have involved comparing measured full-scale pavement test section responses with the predicted ones using the anisotropic aggregate modulus characterization. A simplified approach of obtaining level of anisotropy as an aggregate quality indicator is summarized to indicate use of high quality aggregates with superior properties, including size, gradation and shape, critical to successful pavement designs. Recent efforts as well as the need to develop simplified design and analysis methodologies that will enable optimized use of engineered aggregate structural layers for improved performance and increased use in flexible pavements are also discussed in detail. Copyright ASCE 2008.