The authors review the capabilities of the typical modern airplane for high-altitude operation and present the restrictions of the power plant of the modern airplane which prevent it from delivering its normal output at high altitudes. The paper also discusses the improvements in the aerodynamic characteristics of the airplane as well as changes in the power-plant design and the progress achieved in cruising operation at high altitudes within the limitations imposed by passenger comfort and safety. The authors evaluate further possible gains which may result from an extension of this trend, and examine the engineering problems involved in both the airplane and the engine.