This is the only reference that presents the fundamental physics of piezoelectric sensors in broad scope and close detail. It opens with the elements of phenomenological crystal physics, to develop a fundamental understanding of piezoelectricity. The constitutive equations for elastic and electric field quantities are discussed subsequently, involving both linear and nonlinear material constants. Ferroelectric phenomena in crystals, phase transitions and ferrobielastic hystersis in a-quartz are summarized. A concluding chapter offers a straightforward, easy-to-grasp description of several important piezoelectric materials.