There are now many S- and C-band weather radars around the world that are either dual polarized or soon will be. The most common polarization basis is horizontal and vertical. Dual polarization is accomplished via fast alternate transmission of horizontal and vertical polarization or simultaneous horizontal and vertical polarization transmission. In either case, such radars can measure copolar differential reflectivity (Zdr) and copolar differential phase (Phidp). It has been shown that accurate estimation of these parameters can improve identification and quantification of precipitation. To minimize rainfall estimation error, measurement uncertainty of Zdr should be less than 0.1 dB. Three methods for Zdr calibration are presented and operational implementation is discussed. Concepts are illustrated with data from S-Pol, NCAR's polarimetric S-band radar.