The interfacial properties between carbon fibers and surrounding matrix of a composite are drastically affected by interfacial structure. This structure mainly relates to the surface physico-chemistry of the fiber, which includes its surface chemical groups and microstructure, morphology, surface area, and surface free energy. These properties can be changed by various surface treatments, including various dry and wet oxidation steps, plasma treatment, electrodischarge, and fiber sizing or coating. These methods improve the interfacial properties significantly and synergistically, although each treatment has its specific application area. 85. L. T. Drzal, M.