Like all dielectrics, the permittivity of ferroelectrics shows a distinct frequency dependence. Typical clock frequencies of integrated circuits have long entered a regime where the frequency dependence of basic material parameters like coercive field and remanent polarization can no longer be neglected. At high frequencies hysteresis widens and the coercive field increases, which is of fundamental interest for the extraction of parameters for write- and read-cycles like applied voltage or pulse length.