Chemically and biologically functionalized piezoelectric sensors are attractive alternatives to surface-sensitive transducers due to their surpassing versatility. The fourth volume of the "Springer Series on Chemical Sensors and Biosensors" includes a comprehensive theoretical treatment and 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 and monitoring 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 electrochemical QCM, clearly reveal the advantages of combining multiple techniques to realize new detection schemes on the basis of piezoelectric resonators.