The piezoelectric response of [001] poled domain engineered (1-x)Pb(Mg(13)Nb(23))O(3)-xPbTiO(3) (PMN-PT) crystals was investigated as a function of composition and phase using Rayleigh analysis. The results revealed that the intrinsic (reversible) contribution plays a dominant role in the high piezoelectric activity for PMN-PT crystals. The intrinsic piezoelectric response of the monoclinic (M(C)) PMN-xPT crystals, 0.31</=x</=0.35, exhibited peak values for compositions close to R-M(C) and M(C)-T phase boundaries, however, being less than 2000 pCN. In the rhombohedral phase region, x</=0.30, the intrinsic piezoelectric response was found to increase as the composition approached the rhombohedral-monoclinic (R-M(C)) phase boundary. The maximum piezoelectric response was observed in rhombohedral PMN-0.30PT crystals, being on the order of 2500 pCN. This ultrahigh piezoelectric response was determined to be related to the high shear piezoelectric activity of single domain state, corresponding to an ease in polarization rotation, for compositions close to a morphotropic phase boundary (MPB). The role of monoclinic phase is only to form a MPB with R phase, but not directly contribute to the ultrahigh piezoelectric activity in rhombohedral PMN-0.30PT crystals. The extrinsic contribution to piezoelectric activity was found to be less than 5% for the compositions away from R-M(C) and M(C)-T phase boundaries, due to a stable domain engineered structure. As the composition approached MPBs, the extrinsic contribution increased slightly (<10%), due to the enhanced motion of phase boundaries.