Quartz and other piezoelectric materials have been used as gravimetric mass sensors in both gas and liquid phase corrosion systems. Several different devices have been developed for the studies, including the quartz crystal microbalance (QCM), surface acoustic wave (SAW) devices, plate mode sensors, and Lamb wave oscillators. In each case, an increase in mass of the crystal, or of a film deposited on the crystal, leads to a change in the characteristic frequency of vibration or a change of velocity of propagation of elastic waves in the piezoelectric substrate material. The relationship between the change in mass and the associated change in frequency (or velocity of propagation) is linear over a fairly wide range of operation. This, coupled with the low cost and high mass sensitivity, makes the gravimetric sensors quite attractive to a wide range of applications in corrosion science and engineering. In addition, one may make direct and continuous mass change measurements in flowing gas and liquid streams which make the monitors potentially useful for the chemical process industry.