The paper briefly reviews the evolution of modern aero engines and analyzes the forces which motivate continued technical development, especially the interaction with growth in traffic and aircraft size. The contribution of improved propulsion systems to the economics and regularity of air transport is examined, with particular reference to developments during the past decade. There is some discussion of the environmental factors, particularly noise, raised by civil aviation, the progress already made to deal with them, and the possibilities for the future. The overall benefits of powerplant technical development, particularly as they affect the general and traveling public, are summarized. Examples are drawn from both the United States and European scenes.