The Upper Atmosphere Research Satellite (UARS) is a large (10,000 lbs, 170" diameter, 30' high) NASA/Goddard Space Flight Center spacecraft that will study aspects of the chemistry, dynamics and energy balance of the atmosphere above the troposphere. It will carry ten instruments operating in wavelenghts ranges spanning from the UV to the microwave region. While some of the instruments are only marginally affected by contamination, instruments that are known to be typically sensitive to contamination (like UV solar pointing and cryogenic instruments) will be on the UARS. The potential for cross-contamination is very high. The instruments are being developed by different organizations (each of them having different facilities and operational procedures), will be integrated at the spacecraft contractor site, and will share the same environment during the system performance tests and, obviously, the same flight environments. The goal of the UARS contamination control effort is to allow all the instruments to operate at the desired level of performance, and to achieve this within a non-unlimited budget. Given the mentioned operational scenario and the heterogeneity of the individual instrument requirements, it is easy to appreciate that the control of contamination for the UARS is an extremely complex and challenging task. In this paper, the instrument sensitivities and science requirements are described, and the analytical work leading to the derivation of the contamination control requirements is outlined.