This study was initiated with the general objective of quantifying some of the parameters that govern the stability and rutting characteristics of asphalt-bound materials. Aggregate type and gradation, loading magnitudes and patterns asphalt types and viscosity, are known to affect the stability of an asphalt mixture. The study was conducted in three phases. The first phase was a study of the effects of large stone gradations on the rutting and fatigue characteristics of an asphalt base mixture. The second phase was an in-depth study of the mastic portion (smaller than 3/8-inch) of an asphalt mixture. The third phase was a laboratory analysts of field cores obtained from in-service pavements, and rutting measurements on a number of in-service pavements. It can be concluded tnat large stone base mixtures are less susceptible to rutting than base mixture with finer gradations. Also, large stone base mixture having a polymer modified asphalt cement appeared to have less rutting potential (based on laboratory rutting tests) than an equivalent mixture without a modified asphalt cement. Mastic mixtures that contain manufactured limestone sands have less rutting potential than mixtures that contain natural sands. It is recommended that the percentage of natural sands be limited in the mastic portion of a mixture. From this study, there appears to be no correlation between Marshall stability and resilient modulus when testing field cores. It is concluded that the Marshall test is not a good indicator of rutting behavior or resilient strength.