Carbon fibers are finding increasing application in making various composites with special properties. It is found that such composites have properties that can be markedly effected by the carbon fiber surface. The effects of surface modification by electrochemical and plasma oxidation is discussed and the surface chemical changes described. The interaction of the matrix with the fiber surface is a subtle mixture of physical and chemical effects. The paper discusses work in the author's laboratory using surface studies with core and valence band X-ray photoelectron spectroscopy (XPS) and other techniques.