The fracture properties of selected commercial composite dental restorative materials and a model composite system were studied to determine the influences of the reinforcing phase, exposure to water, and particle/polymer adhesion on crack propagation. The content of inorganic fillers ranged from 36 to 62 volume percent. In the model system the polymer phase approximated that of the commercial products, a constant size distribution of quartz fillers was used, and polymer/particle adhesion was varied. The double torsion test method was employed to measure relationships between applied stress intensity factor and velocity of crack propagation during stable crack growth. In all systems, cracks propagated through regions of high stress concentration at the low end of the velocity range studied (10−7 m/sec to 10−3 m/sec). Wet materials fractured at lower stress intensities than dry materials at all velocities. At high velocities unstable (stick-slip) growth occurred in dry materials with strong filler/matrix interfaces and in wet specimens with initially strong interfaces and less than 41 volume percent filler. In wet conditions, materials with poorly bonded fillers fractured by slow crack growth at stress intensities 10% to 30% below the levels of composites with strong interfaces.