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This paper reviews recent theoretical and experimental research on cross polarization that occurs during precipitation on terrestrial microwave links. Consideration is first given to the physical mechanisms, such as raindrop canting, that cause precipitation depolarization. Coherent rain depolarization models are then discussed and recent investigations of incoherent depolarization effects summarized. Particular attention is given to semi-empirical models since they can be used most readily for statistical predictions in design applications. ‘Model oriented’ cross polarization measurements aimed at characterizing the physical properties of the depolarizing medium for use in model development are also given particular attention. Effective parameters of the raindrop canting angle distribution derived from these measurements are tabulated. Finally, some mention is made of the relative importance of rain depolarization and attenuation in causing outages on digital radio systems.