Three ceramic preparations of lead scandium tantalate were chosen for the present study because they exhibit three different relaxer-type dielectric response behaviours. Two preparations, made in lead-rich atmospheres, so as to conserve virtually 100% occupancy of the perovskite (ABO(3)) A sites, were annealed or quenched so as to yield extreme values for the long-range order parameter (s) for (Sc, Ta) ordering on the perovskite B site;thus s = 0.93 and 0.10 for pst-o and pst-d, respectively. The third preparation, made in the classical way in the expectation that there would be significant levels of Pb atom vacancies, allowed earlier literature results for the dielectric response to be reproduced (sample pst-def; s = 0.20). Results of high-resolution and dark-field transmission electron microscopic studies and electrical property measurements are compared in order to elucidate, as directly as possible, the structure-property relationships. In particular the chemical and polar domain structures are revealed and the unusually high density of chemical domain wall (CDW) and polar domain wall (PDW) structures characterized. New types of extended defects are found in the Pb-deficient preparation, including edge, screw and helical dislocations and small (ca, 2 nm diameter) void-like defects. There is also HRTEM evidence for atomic disorder, probably due to Pb site vacancies. The different degrees of ordering of the B-site cations Sc and Ta were also characterized by HRTEM, as evidenced by chemical domain textures. Direct evidence for Pb-atom displacements was obtained for pst-o and pst-d, allowing polar nanodomain textures to be examined at atomic resolution and the local direction and sense of the polarization vector to be characterized. The results of Monte Carlo and next-nearest-neighbour Ising model simulations for the evolution of the chemical domain textures are compared with the experimental dark-field images. This allows several distinct types of chemical defect clusters to be characterized, the existence of both charged and neutral defects are implied by the simulations. A: discussion of the role played by such defects in producing random local fields and finally determining the frequency dependence of the dielectric response completes the paper.