Abstract : The relation between ferroelectricity and pyroelectricity in the perovskites is explored using Devonshire's phenomenological theory of ferroelectricity and a new microscopic theory obtained by generalizing the conventional molecular field theory. The Devonshire formalism yields results for the pyroelectric coefficient and the figures of merit for pyroelectric detectors in terms of the parameters which characterize the dielectric behavior and the ferroelectric phase transition. A simple thermodynamic relation for the figure of merit is also derived. The molecular field theory is generalized in a novel way which permits it to describe first as well as second-order phase transitions. Good agreement with experiment is obtained for BaTiO3. The new theory assumes the existence of a disordered array of dipoles in the perovskites, and evidence for this disorder is obtained from measurements of Raman scattering in KNbO3.