many naturally occurring structures are, in fact, composite materials with functionally graded mechanical properties. in manufacturing, achieving such graded properties is critical when joining two materials of significantly different elastic moduli, thermal or electronic characteristics. One assumes that similar imperatives are at play in the profusion of biological examples of gradients. Two gradient biosystems, in particular, have been the focus of our attention: These are mussel byssal threads and polychaete jaws. Mussel byssal threads are composites consisting of anisotropically packed discontinuous fibers in an amorphous matrix all coated by a protective cuticle. The threads are produced by process resembling reaction injection molding and imparted with a longitudinal gradient of mechanical properties ranging from stiff and tough distally to extensible and rubbery proximally. Collagen is the principal tensile polymer as determined by fiber x-ray diffraction, however, the mechanical properties of typical tendon are poorly matched with those of byssal threads.