Microelectromechanical systems (MEMS) incorporating piezoelectric layers provide active transduction between electrical and mechanical energy, which enables highly sensitive sensors and low-voltage driven actuators surpassing the passive operation of electrostatic MEMS. Several different piezoelectric materials have been successfully integrated into MEMS structures, most notably Pb(Zr,Ti)O3. Piezoelectric materials with larger piezoelectric response, such as the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT), would enable further miniaturization. However, this has long been hampered by the difficulties in the synthesis of these materials. This article reviews recent successes not only in synthesizing high-quality epitaxial PMN-PT heterostructures on Si, but also in fabricating PMN-PT microcantilevers, which retain the piezoelectric properties of bulk PMN-PT single crystals. These epitaxial heterostructures provide a platform to build MEMS and nanoelectromechanical system devices that function with large displacement at low drive voltages, such as ultrasound medical imagers, micro-fluidic control, piezotronics, and energy harvesting.