Approximate and exact mathematical techniques for the calculation of the scattering properties of a single ice particle are first reviewed. Then, for dealing with practical applications such as radar meteorology or radio wave transmission along earth-space links, the problem of the scattering by an assembly of particles is examined. This problem involves the knowledge of the particle size distribution and of the orienting mechanisms which can affect the ice crystals in the clouds. Although the T matrix formalism has been successfully used in a greater range of scattering problems than the other techniques, the Rayleigh approximation may be used with confidence for calculating the cross-polarization properties of radio waves at frequencies up to at least 25 GHz. Alignment of ice particles is not expected to occur in high-altitude cirrus clouds while alignment is observed in strong thunderstorms. Thus different models are to be used in order to predict the cross polarization along earth-space links.