The quartz crystal microbalance (QCM) is a sensor device for measuring tiny amounts of mass accumulating on or dissolving from a surface. In the widespread version, the resonance frequency (above 1?MHz) of a piezoelectric disk resonator oscillating in the so-called thickness shear mode (TSM) is altered by the amount of mass on its sensitive surface. In thin film deposition processes it is used to measure the amount of deposited mass down to atomic monolayers. The high sensitivity and the possibility to functionalize the surface motivated their application in biochemistry. Operating the QCM in a liquid environment is possible but requires a careful analysis to account for the fluid effect. In the case where, in addition to the resonance frequency, the damping is measured, the method is called QCM-D. What is truly measured is the acoustic impedance at the sensitive surface, which corresponds to the attached mass only in the case of a thin rigid film. In all other cases, the wave propagation into the medium has to be considered in the data evaluation. A further extension is the measurement at overtones, which allows the determination of material properties of viscoelastic layers.