Thin film piezoelectrics are important for microelectromechanical systems (MEMS) due to their ability to generate large motions with low hysteresis, high energy densities, and high sensitivity. This paper reviews factors affecting piezoelectric response, focusing on materials like ZnO and AlN, where film orientation is crucial. AlN has high electrical resistivity, is CMOS compatible, and has a high frequency constant, making it suitable for resonators. Ferroelectric films, such as PZT, offer higher piezoelectric response, enabling lower voltage actuators and sensors. PZT films with random orientation have e31,f coefficients of about -7 C/m² at the morphotropic phase boundary. Orientation, composition, grain size, defect chemistry, and mechanical conditions affect piezoelectric coefficients. The highest responses are in {001} oriented rhombohedrally-distorted perovskites, with e31,f coefficients ranging from -12 to -27 C/m². MEMS applications benefit from piezoelectrics due to their high-frequency resonant structures, low power consumption, and ability to perform large amplitude actuation with low voltage and hysteresis. Piezoelectricity scales well with size, making it suitable for MEMS. They are CMOS compatible and can provide electrical signals for sensing or actuation. These advantages make piezoelectric films practical for applications like filters, micromotors, micropumps, and accelerometers. Piezoelectric coefficients describe the relationship between electric field, strain, and stress. They are components of a third-rank tensor, with d and e coefficients related through the stiffness tensor. Piezoelectricity occurs in crystals without a center of inversion. Ferroelectric materials allow reorientation of the polar axis, enabling internal polarization and net piezoelectric effect. Poling aligns polarization parallel to the poling field, enhancing piezoelectric response. In vertically stacked capacitor structures, direction 3 is perpendicular to the film plane. The d33 coefficient describes the piezoelectric effect in these materials.Thin film piezoelectrics are important for microelectromechanical systems (MEMS) due to their ability to generate large motions with low hysteresis, high energy densities, and high sensitivity. This paper reviews factors affecting piezoelectric response, focusing on materials like ZnO and AlN, where film orientation is crucial. AlN has high electrical resistivity, is CMOS compatible, and has a high frequency constant, making it suitable for resonators. Ferroelectric films, such as PZT, offer higher piezoelectric response, enabling lower voltage actuators and sensors. PZT films with random orientation have e31,f coefficients of about -7 C/m² at the morphotropic phase boundary. Orientation, composition, grain size, defect chemistry, and mechanical conditions affect piezoelectric coefficients. The highest responses are in {001} oriented rhombohedrally-distorted perovskites, with e31,f coefficients ranging from -12 to -27 C/m². MEMS applications benefit from piezoelectrics due to their high-frequency resonant structures, low power consumption, and ability to perform large amplitude actuation with low voltage and hysteresis. Piezoelectricity scales well with size, making it suitable for MEMS. They are CMOS compatible and can provide electrical signals for sensing or actuation. These advantages make piezoelectric films practical for applications like filters, micromotors, micropumps, and accelerometers. Piezoelectric coefficients describe the relationship between electric field, strain, and stress. They are components of a third-rank tensor, with d and e coefficients related through the stiffness tensor. Piezoelectricity occurs in crystals without a center of inversion. Ferroelectric materials allow reorientation of the polar axis, enabling internal polarization and net piezoelectric effect. Poling aligns polarization parallel to the poling field, enhancing piezoelectric response. In vertically stacked capacitor structures, direction 3 is perpendicular to the film plane. The d33 coefficient describes the piezoelectric effect in these materials.