The mechanism of wear of commercial and experimental formulations of composite restorative resins will be studied in the laboratory by a two-body abrasion test and a single-pass sliding test. Also, the wear of two commercial composites (one with glass filler and one with quartz filler) and an amalgam will be evaluated clinically in Class I restorations in human patients over a three-year period. In the laboratory phase, the effect of the resin matrix, silane coupling agent and inorganic reinforcing agent on the abrasion resistance of experimental composites will be studied. The objectives of the clinical study are to evaluate a methodology for quantitative assessment of clinical abrasion in posterior teeth, to correlate clinical abrasion with laboratory studies, and to obtain a comparative analysis of the wear rate of three commercially available restorative materials. The information should be useful in improving the wear resistance of composite restorative materials.