This paper describes a rational basis for making tradeoff decisions on cost and weight for structures made of various materials. Unit cost and empty weight data have been gathered for a variety of consumer products and of aircraft. As examples illustrating potential applications of filamentary composites, the cost/weight tradeoff ratios were calculated for three typical aircraft structural components: a landing-gear cantilever-spring strut, an I-beam, and a shear panel. In addition, weight analyses of fuselage structures were made for designs using a variety of materials and these structural concepts: monocoque, sandwich, and ring-and-stringer-stiffened cylinders. With the cost reductions anticipated in the next three or four years for graphite and boron filamentary composites, fiber composites hold considerable promise for general-aviation aircraft.