Intended as an introduction and reference for materials scientists and physicists, this book treats the fundamental physics of piezoleletric sensors. It begins with the elements of phenomenological crystal physics to develop a fundamental understanding of piezoelectricity. Subsequently the constitutive equations for elastic and electric field quantities are discussed involving both linear and nonlinear material constants. Ferroelectric phenomena in crystals, phase transitions and ferrobielastic hystersis in a-quartz are summarized. It concludes with a chapter that explains several important piezoelectric materials in a straight-forward and easy to grasp way.