Interfaces, such as mortar-aggregate interfaces and cement paste-fiber interfaces, affect the mechanical behavior of concrete composites. Characterization of interfacial behavior is needed to study the role of the interfaces on the global behavior of concrete composites as a basis for the development of high-performance cementitious materials. In this paper, an interface fracture mechanics-based methodology is presented to assess the fracture toughness of mortar-aggregate interfaces. First, sandwich specimens used to develop the fracture toughness curves of mortar-aggregate interfaces are described. Then, an experimental and numerical simulation study is presented of a two-phase beam composite model consisting of a mortar matrix and an aggregate inclusion to investigate the crack penetration versus crack deflection scenarios in the interfacial regions. The results from the composite model study are correlated with those obtained from testing sandwich specimens.