Polarization elements are inadequately characterized for many applications. For retarders, usually only the retardance is specified. For polarizers, usually only the two principal transmittances or the extinction ratio are given. For polarization elements used in critical applications, this level of characterization is woefully inadequate. Defects of polarization elements are described, and the Mueller calculus suggested as the most appropriate measure of performance. Examples of the characterization of polarization elements as a function of angle of incidence, and as a function of wavelength are provided.