An improved understanding of pavement permanent deformation accumulation and factors contributing to the base course rutting behavior is important for pavement design, construction, maintenance and modeling. This paper provides laboratory test results and predictive rutting model analyses from an ongoing research project at the University of Illinois at Urbana-Champaign (UIUC). Moisture-density, monotonic triaxial shear and repeated-load triaxial tests were conducted on four aggregate materials having different gradations. A rutting model developed at UIUC predicts permanent strains as a function of number of load repetitions, applied deviator stress, and mobilized Shear Stress (to strength) Ratio (SSR). The model was developed based on the permanent strain trends measured on aggregate specimens from laboratory tests. Applied stress states and SSR levels were found to be the most important in controlling the development of permanent deformation at typically the most critical low confinement conditions.