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-in.) of an asphalt mixture. The third phase was a laboratory analysis of field cores obtained from in-service pavements, and rutting measurements on a number of in-service pavements.