Relaxor-based ferroelectric single crystal solid solution (1-x) Pb (Zn1/3Nb2/3) O3-xPbTiO3, abbreviated (1-x) PZN-x PT, binary system with compositions near a morphotropic phase boundary (MPB) have attracted much attention for applications in high performance actuator, non-destructive testing and ultrasonic devices, making use of its superior piezoelectric properties. 0.91Pb (Zn1/3Nb2/3) O3-0.09PbTiO3 (91PZN-9PT) single crystal exhibits excellent piezoelectric constant (d33~2500pC/N) and high electromechanical coupling coefficient (k33~94%) in the rhombohedral(R) phase in engineered domain state. A pure compound, lead titanate, can display a MPB under pressure.The electric field-induced domain wall movement and the high-density domain walls associated with the nanotwin related domains to have a significant contribution to the superior piezoelectric properties near the MPB. In this book, the electric field-induced switching of nanotwin related domains was investigated in 91PZN-9PT single crystals by scanning force microscopy in the piezoresponse mode (PFM).