Engineering development has always been restricted by the limitations of structural materials. In the development of large fan stage compressor blades, material of high strength to density and modulus to density ratios have been required. This paper discusses use of titanium in the 170 and 190 ksi yield strength alloys in these applications. Materials, procedures, and results of the Pratt & Whitney development program are discussed.The Ti-6Al-6V-2Sn alloy appears most promising in meeting the desired 170 ksi yield strength while maintaining reasonable fracture toughness. The relationship between toughness, fatigue, and other mechanical properties is incompletely understood. However, with high tensile preloads, a strong positive correlation between toughness and fatigue strength seems quite probable.