A series of polymer/fiberglass composites were fabricated with different random glass mat reinforcement materials in a common resin matrix. Digital image analysis was used to characterize the structure in terms of the individual glass fibers, the fiber bundles (termed tows), and the distribution of the tows within the composite. Specialized grain reconstruction programs were used to characterize both intertow and tow dimensional attributes. The microstructural characteristics were then related to various mechanical properties of the different composite specimens, such as tensile strength and modulus (measured under shear and elastic conditions). Computer-aided-analysis microscopy can be shown to be important in correlating micro- and macrostructure to the measured properties of polymer/glass composite systems.