Interfaces, such as mortar-aggregate interfaces and cement matrix-fiber interfaces, affect the mechanical behavior of concrete composites. Significant considerations in understanding the mechanical behavior of concrete are the nature of the deformation and failure of these interfaces and the interaction between the constituent elements of the composite. Development of advanced concrete materials with improved toughness and durability requires a fundamental understanding of the behavior of the interfaces which are intrinsic to the concrete composite. Therefore, there is a needed to characterize the interfacial behavior and to study the role of the interfaces on the global material behavior as a basis for the development of high performance cementitious materials. To address this need, ACI International produced the specialized publication based on the technical papers presented during a special session on fracture mechanics.