The fracture behavior of concrete is often attributed to a fracture process zone. This fracture process zone manifests itself in the the form of nonlinear stress-strain behavior, post peak strain softening, size effect, and numerous toughening mechanisms. Features of the fracture process zone include arrays of microcracks, aggregate interlocking, crack bridging, and grain boundary sliding friction. From a material modeling standpoint, properties of the fracture process zone must be known in order to accurately predict the response of the material to stress. Since the fracture process zone characteristics are critical to material properties, a better understanding of those characteristics will lead to a better understanding of overall performance.