Recent studies of the mechanism of piezoelectricity in PZT and related materials are reviewed and complemented by new analyses based on the Landau-Ginzburg-Devonshire theory of ferroelectricity. Particular attention is given to the nature of the morphotropic phase boundary between the tetragonal and rhombohedral perovskite phases and the accompanying peak in the piezoelectric coefficient. The importance of the changes in angular dependence of single crystal piezoelectric coefficients as a function of composition is highlighted together with the concept of field-induced rotation of the polarization in the (110) plane. It is shown that introducing the tendency to form monoclinic phases enhances this phenomenon. The model that the monoclinic phase in PZT is due to the condensation of local disorder in the polar cation displacements from the macroscopic tetragonal and rhombohedral phases is examined in some detail using statistical analyses of the Zr/Ti conformation. Whilst the concept of monoclinic nano-domains is not inconsistent with statistically random distributions, it is argued that some ordering of the B-site cations may be required to enable the transformation to a macroscopically observable phase. The implications of this model on the contribution of polarization rotation to piezoelectricity in PZT are discussed. (c) 2006 Springer Science + Business Media, Inc.