Flight simulation has become an indispensable portion of the aircraft development process. It can be performed either on the ground, in a now familiar ground based simulator, or in actual flight, using a specially developed variable stability aircraft, commonly called an in-flight simulator. Ground based and airborne simulation each offer unique advantages and limitations. For a specific task, one type may not be the best choice every time. This paper discusses major capabilities and limitations of ground based and in-flight simulation. The unique role of in-flight simulation for test pilot training is covered.