The tensile properties of carbon fibers are usually determined by one of two methods, the single filament tensile test and the composite strand tensile test. There is a great variation in the tensile strengths of filaments, both along and between given yarns within a spool of carbon fibers, as well as variation between spools and lot numbers. The presence of microfibrils has been blamed for the low compressive strength, as microfibrils were observed in Scanning electron microscopy (SEM) photomicrographs following compressive failure. Emphasis for space materials will focus on minimization of the axial coefficient of thermal expansion and on increasing thermal conductivity, the former for stable structures, as equipment is subjected to large temperature variations in orbit, and the latter for maintaining heat balances within the equipment. The structure-property relationships depend upon the size, orientation, and perfection of the crystalline regions within the microfibrils and the interaction between microfibrils.