This chapter deals with the techniques of surface characterization of carbon fibers and determination of the shear strength between fiber and matrix. A review of earlier methods of surface characterization can be found in McKee and Mimeault. The shear strength between fiber and matrix is, in part, dependent on the surface properties and can be estimated either with model systems or composite specimens. The characterization of the surface of carbon fibers by infrared spectroscopy is difficult due to severe scattering by the fibers and the high absorption coefficient of the black material. An alternate method for determining surface energy of carbon fibers, which avoids some of the complications of the single liquid method but gives rise to other problems is described by Schultz et al. The surface energy of a solid can also be determined by inverse gas chromatography (IGC) where the column packing is the item of interest and various vapors are used as probe molecules.