This chapter begins with a detailed introduction to composite interfaces. The focus is on fiber-matrix interfaces. Before embarking on an account of interfacial phenomena, theories of adhesion in metallic, ceramic, plastic, biopolymers and hybrids are documented and illustrated starting from pioneering investigations. Case studies of reinforcement-matrix interfaces are provided. Available surface technologies that enhance interfacial adhesion between the reinforcement and the matrix are provided and an extended list of techniques and materials is given. Surface and interface characterization through microscopic and spectroscopic methods is added as an illustrative section that educates the reader about structure property correlations in composite surfaces and their interfaces and assists an understanding of the correlations of their physical, chemical, and mechanical evaluation. The highlight of the chapter is a complete rendering of the pioneering and advanced methods conducted to evaluate the interfacial properties of metallic, ceramic, polymeric, and hybrid composite systems. The chapter also familiarizes the reader in the basic essentials of micro-, meso-, macromechanics and the mechanisms associated with measuring the interfacial strength, stiffness, and toughness of fiber-polymer systems. A special focus on fractography and the progressive fracture of the fiber-matrix interface is included to provide the reader with an understanding of fracture processes and failure sequences in the interfaces. The chapter ends with an overview of novel nanoscale, biological, multiscale, and macrocomposite interfaces and their evaluation. Finally, the summary and conclusions are succeeded by a short treatise on futuristic directions, suggestions, and implications regarding further research. © 2019 Elsevier Ltd. All rights reserved.