This chapter focuses on aerothermodynamic improvements to aircraft engine systems that were enabled through National Aeronautics and Space Administration (NASA) research efforts. Interest in aircraft fuel efficiency increased dramatically in the 1970s because of the sharp rise in jet fuel prices and their effect on the airline industry. NASA has directly contributed to computational fluid dynamics analysis improvement through development of NASA in-house turbomachinery codes that have contributed to the body of knowledge in the field. It has been collaborating with General Electric Aviation and the Federal Aviation Administration to explore the design space for lower noise while maintaining the high propulsive efficiency from a counter-rotating open-rotor system. The ultrahigh-bypass engine technology associated with the first-generation Pratt & Whitney geared-turbofan was close to reaching NASA's n+1 noise and fuel burn reduction goals, but additional technologies are needed to achieve the n+2 goals.