Electric-field-induced phase transitions have been evidenced by macroscopic strain measurements at temperatures between 25 degrees C and 100 degrees C in [001](C)-poled (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) [(PMN-xPT);x=0.25,0.305,0.31] and (1-x)Pb(Zn1/3Nb2/3)O-3-xPbTiO(3) [(PZN-xPT);x=0.05,0.065,0.085] single crystals. Such measurements provide a convenient way of ascertaining thermal and electrical phase stabilities over a range of compositions and give direct evidence for first-order phase transitions. A pseudorhombohedral (M-A)-pseudo-orthorhombic (M-C)-tetragonal (T) polarization rotation path is evidenced by two first-order-like, hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT, PMN-30.5PT, and PMN-31PT whereas, in PMN-25PT, a single first-order-like M-A-T transition is observed. This agrees well with in situ structural studies reported elsewhere. Electric-field-temperature (E-T) phase diagrams are constructed showing general trends for M-A, M-C, and T phase stabilities for varying temperatures and electric fields in poled samples over the given range of compositions. The complex question of whether the M-A and M-C states constitute true phases, or rather piezoelectrically distorted versions of their rhombohedral (R) and orthorhombic (O) parents, is discussed. Finally, stress-induced phase transitions are evidenced in [001](C)-poled PZN-4.5PT by application of a moderate compressive stress (< 100 MPa) both along and perpendicularly to the poling direction (longitudinal and transverse modes, respectively). The rotation path is likely R-M-B-O, via a first-order, hysteretic rotation within the M-B monoclinic plane. The results are presented alongside a thorough review of previously reported electric-field-induced and stress-induced phase transitions in PMN-xPT and PZN-xPT.