Much attention is drawn to the fundamental research and the applications of the relaxor-based ferroelectrics for the next generation of piezoelectric devices. Mass production of Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMNT) and ternary xPb(In1/2Nb1/2)O3–yPb(Mg1/3Nb2/3)O3–zPbTiO3 (PIMNT) single crystals with a Bridgman method was reported. The piezoelectric, pyroelectric, and electric–optical performances of PMNT single crystals are much more excellent than traditional ceramics with the composition near morphotropic phase boundaries. Compared with PMNT crystal, PIMNT crystals can be used in a more broad range of device applications due to their high Curie temperature Tc and large coercive fields Ec. A relaxor-based Na0.5Bi0.5TiO3–xBaTiO3 (NBBT) crystal grown by the top seeded solution growth method was also investigated to find a lead-free material with better piezoelectric performance. The properties of these relaxor-based single crystals were summarized. The mechanism of the enhanced piezoelectric response is discussed, based on the phase transitions induced by the variations of the temperature, pressure, composition, and electric field.