The advantages of ground-based simulation relative to flight test are discussed. These include: lower cost, better control of environmental factors, ability to investigate hazardous situations, completely known dynamics, comprehensive measurements and measurement accuracy, repeatability, ease of making changes and comparisons, and early availability. The disadvantages and many practical problems are associated with modeling errors and unrealistic pilot behavior. The modeling features discussed are: the aerodynamic model, modeling of atmospheric disturbances, ground effects, landing gear dynamics, sampled-data effects, and extrapolation to very low speeds. Unrealistic pilot behavior results from distortions in the visual, motion, and aural cues, and from differences in psychological factors. Practical problems in each of these areas are discussed.