This study was initiated to investigate some potential benefits of polymer modification on dense-graded, heavy- duty pavement mixtures, The following polymers were used to modify asphalt binders and mixtures: an ethyl vinyl acetate (EVA), a low-density polyethylene (LDPE), a styrene butadiene styrene (SBS), an hydrogenated SBS, and a finely ground tire rubber. Each of the polymers was blended with one or more of the following asphalt cements: an AC-10 and three sources of AC-20. Both conventional and recently-developed test methods were performed on the binders and mixtures to study the effects of the polymer modifiers at both low temperatures and high temperatures. Standard Marshall mixture design procedures did not optimize the mixtures containing tire rubber and dense-graded crushed limestone. The effect of polymers on low-temperature properties was dependent on the modifier/polymer combination. Ground tire rubber was particularly effective at these low temperatures. The high-temperature performance of an AC-20 mixture, produced with crushed gravel, was improved by modification with an SBS.