Polarimetric radar measurements can be made either coherently, by measuring covariances of the various elements of the complex scattering matrix, or noncoherently, by measuring a series of scattered Stokes vectors associated with a set of transmit polarizations. The coherent approach can be difficult to apply at millimeter wavelengths, where rapid decorrelation of the scattered signal can cause errors in the measured covariance terms. The noncoherent method is not adversely affected by rapid decorrelation but requires a more complex transmitter to generate multiple polarizations. To compare these two techniques, a 95-GHz polarimetric radar was recently modified to simultaneously measure the Mueller matrix using the coherent and noncoherent approaches. Backscatter measurements of foliage are presented showing the conditions under which the coherent measurements are in error. Finally, we show how to reduce motion-induced errors associated with the coherent technique when sampling a rapidly decorrelating scene. ¿ American Geophysical Union 1996