The intent of this project is to establish the fracture characteristics of a hybrid composite in the ratio of 1/9 microfill to small particle filler and the corresponding unfilled Bis-GMA/ TEDMA resin. This approach is based on the experimental and clinical results which demonstrate that dental composite restorative materials when subject to aging in the oral cavity and the forces of mastication can fail in fatigue due to the effects of stress corrosion, microcracking and particle separation. The experimental procedures consist of fatiguing notched specimens n distilled water at 37oC under fatigue flexure (three point loading). The fracture process will be observed by means of a long range optical microscope. Fracture surface characterization and damage analysis will be carried out on the fractured specimens using principals of quantitative stereology which are widely employed in materials science. The analytical study is aimed at modeling the observed phenomena using principles of fracture mechanics with the theory of statistics incorporated to analyze the scatter of the fracture data. The work of this project attempts to directly link the microscopic measures of fracture initiation and propagation to microstructural variables and ultimately to processing parameters. This knowledge can be used to design better materials as well as longer lasting and more economical dental composites. Alternatively, this approach will provide greater understanding of the fracture behavior of dental composites and the relationship to microstructure under mechanical and environmental loadings. Furthermore, the results of this work will allow us to test the validity and reliability of the techniques employed in engineering to predict the life-time and fracture toughness of dental composites.