The differential scattering properties of classes of hydrometeors at linear orthogonal polarizations provide potentially important differences which may be exploited for radar measurements of precipitation. The ratio of the reflectivity at horizontal (ZH) and vertical (Zv) polarizations may be combined with other radar measurements such as absolute reflectivity and differential phase shift to determine drop size distributions. Previous model calculations of differential reflectivity ZDR = 10 log (ZH/Zy) were made using Gans' theory of scattering by spheroids. New calculations, using Waterman's T-matrix method, are in excellent agreement with Gans' theory for raindrops at S-band wavelengths. The ZDR-differential phase shift combination, which need not depend on any absolute measurements, is shown to be a reasonable radar method for measuring two-parameter raindrop size distributions. Finally, calculations of ZDR for oblate hailstones indicate the possibility of uniquely detecting large, dry hail when the hail is falling with preferred orientation.