Polybenzoxazine synthesized by the ring-opening polymerization of cyclic benzoxazine monomer has been developed as a new type of phenolic resin. The monomers are easily prepared from phenols, amines and formaldehyde. We report here the performance improvement of polybenzoxazines by the molecular design of monomers, for instance, monomers having crosslinkable units, high molecular weight polymeric monomers, and liquid crystalline monomers. The wide variations of raw materials, phenols and amines, allow tremendous molecular-design flexibility for the cyclic monomers. Other approaches for performance enhancement of the polybenzoxazine, including polymer alloys and hybrids with inorganics, are also reviewed.