This paper describes the results of parametric design studies of the application of filamentary composite materials in the structure of high-subsonic-speed transport aircraft. System costs and weight savings are presented as a function of percent utilization of composite materials from zero to 80%. The weight savings potential of composites for direct material substitution and for resized aircraft show gains of up to 25 and 50%, respectively. The state-of-the-art in structural design, analysis, fabrication, and test is discussed. Structural design concepts are shown and test validation is given, along with cost analyses.