In developing a modern commercial aircraft, the airframe and engine manufacturers work as a team to develop the propulsion system and verify the high standards of safety, reliability, and performance the airlines and the public expect. An extremely important part of this undertaking is the testing of the propulsion system components and finally the entire system in realistic simulated operating conditions before the airplane goes into service.With each new airplane system, the design requirements tend to become more demanding and the development and proof testing more exacting to assure that the test simulation of the critical operating environment is correct and that the instrumentation and data systems are adequate to accurately and quickly obtain the required information. Careful judgment and expert ingenuity are required to obtain the required data in a timely and cost-effective manner so as to minimize the test time required. An overview is provided of the airframe manufacturers' viewpoint in the development testing which leads to a new aircraft, including scale model, ground, and flight testing.Major areas which are becoming more critical with each generation of aircraft are discussed in more detail; noise reduction and inlet-engine-exhaust system compatibility being two. Examples of problems which have been faced in recent subsonic airplanes and some of particular interest in advanced aircraft, such as the SST, are discussed.Specific simulation and data acquisition problems, test techniques, and examples of the types of data which are currently used are presented. Finally, the improvements which will be required for more advanced aircraft systems are discussed.