This paper discusses the mechanisms that can contribute to the enhanced longitudinal piezoelectric effect along nonpolar directions in perovskite crystals, such as BaTiO₃, PbTiO₃, KNbO₃, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ and Pb(Zn_1/2Nb_2/3)O₃-PbTiO₃. Piezoelectric anisotropy is discussed in relation to temperature induced phase transitions, compositional variation in solid solutions with morphotropic phase boundaries, applied electric fields, the domain wall structure and domain wall displacement.