Strategies are presented to enhance the mechanical properties of a carbon/epoxy composite, by modification of the interface. Several treatments were applied to the fibers, to alter the surface energy and thus improve the wetting by the resin. Among these are fiber desizing, oxidation and surfactants. Nanoparticles and nanofibers were introduced at the interface to serve as "anchors" between the fiber and the matrix, in an attempt to strengthen the fiber/matrix interaction. A reactive hot melt sizing system was also developed, and compared to commercial sizing. The fiber-matrix interphase was evaluated with the micro-droplet pull out test, the fragmentation test, contact angle and surface free energy (SFE) measurements. A strong link was shown between the polar SFE and the interfacial shear strength (IFSS). The polar SFE may help predict the interfacial mechanical properties of the composite. The new reactive sizing formulation, combined with surface functionalization showed over 50% increase in IFSS.