This application proposes to study the relationships between structure, mechanical properties, and clinical performance of restorative dental composites. The studies in Aim 1 will produce experimental formulations and measure various laboratory properties (fracture toughness, elastic modulus, transverse strength, hardness, and degree of cure). Aim 2 is to conduct a clinical trial with the experimental formulations. Aim 3 is to retrieve the restorations from the clinical study after three years, and determine the extent of the subsurface breakdown and its relationship to the previously measured mechanical properties and abrasive wear, and to correlate changes in the degree of cure and alteration in the polymer matrix during in vivo and in vitro aging. Aim 4 is to compare fracture toughness results obtained with the various methods. The last Aim is to establish a correlation between fracture toughness and the phenomenon of surface cracking.