Chemically and biologically functionalized piezoelectric sensors are attractive alternatives to surface sensitive transducers due to their surpassing versatility. The fourth volume of Springer Series on Chemical Sensors and Biosensors includes a comprehensive theoretical treatment and the current state-of-the art applications of the quartz crystal microbalance (QCM). Interface circuits and the study of viscoelasticity and micromechanics as well as surface roughness with the QCM are discussed. The broad field of analytical applications of piezoelectric sensors is covered, which ranges from nucleic acid detection, immunosensors, protein-membrane interactions, monitoring of cells by imprinted polymers to the viscoelastic response of living mammalian cells on QCM-resonators. Sophisticated derivatives of the classical QCM such as rupture event scanning, the use of extraordinary high frequency crystals, and the electrochemical QCM clearly reveal the advantages of combining multiple techniques to realize new detection schemes on the basis of piezoelectric resonators.