Starting from known features of the mechanical behavior of macromolecular materials, and from how reinforcement affords compound properties, a thesis is advanced how composite performance and endurance may be based on a fine tuning of the properties of reinforcer, interphase, and matrix. The importance of their relative moduli and extensibilities and the critical role of the interphase are emphasized. It is proposed that the latter's ability to strain harden is crucial. Thus, the interphase should be only slightly crosslinked, its wetting of filler and matrix aided only by a small extent of interfacial covalent-bonding, and its modulus be raised by the addition of well-submicron size reinforcing filler. In this context, the meaning of some published data is discussed.