INTRODUCTIONSilicon carbide (SiC) ceramic matrix composites (CMCs) reinforced by continuous-length, high performance, smalldiameter (∼10 to 15 μm), polycrystalline SiC fibers are considered enabling materials for a variety of advanced applications where lightweight reusable structural materials are required to operate for long time periods within extreme high temperature environments. Today the first generations of SiC/SiC CMCs with thermo-structural capability to ∼1250 • C are being introduced into the hot-section components of military and commercial gas turbine engines [1,2]. In comparison to superalloy metallic components, which at best operate at as high as ∼1100 • C, these CMC components will not only offer reduced engine weight, but also reduced component cooling-air requirements. Reduction in cooling air would then have the additional engine benefits of improved thrust-to-weight ratio, reduced fuel burn, and reduced harmful exhaust emissions. In order to seek further increases in these engine benefits, research has been ongoing within NASA aimed at developing SiC/SiC CMCs with even higher structural reliability and temperature capability.The objective of this chapter is to detail, both fundamentally and practically, the key advances from recent SiC/SiC CMC research activities within NASA. Although these advances are primarily still under development and optimization, they do show various degrees of improvement over the materials and processes currently being implemented for SiC/SiC engine components. To this end, Section 7.2 presents information on the general constituent materials and process requirements for structurally reliable high temperature SiC/SiC engine components. Section 7.3 then discusses the primary fabrication routes currently being employed for production-ready SiC/SiC components in terms of their benefits and limitations. Section 7.4 then details recent advancements in the materials and processes for the SiC fiber, the fiber interfacial coating, the fiber geometry or architecture, the SiC-based matrix, and in the design methods for assembling these microstructural constituents into higher temperature and higher performance SiC/SiC composites and components. Where possible, CMC property data will be presented and analyzed that directly compares the materials and processes of the advanced approaches against those of current approaches. This property comparison will focus primarily on such key CMC properties as in-plane and thru-thickness tensile strength, thermal conductivity, creep resistance, and rupture behavior since it is these properties that directly control the CMC structural and upper temperature capability. Finally, Section 7.5 summarizes these results into current design guidelines for achieving SiC/SiC microstructures with improved intrinsic thermo-structural capability, and then discusses potential issues, such as thermo-mechanical shock, environmental attack, and creep-related residual stress development, which still require further research to understand whether they will be ...