The book "Piezoelectric Sensors" from the Springer series on chemical sensors and biosensors provides a comprehensive theoretical treatment as well as current applications of piezoelectric sensors, particularly the Quartz Crystal Microbalance (QCM). It highlights the versatility of chemically and biologically functionalized piezoelectric sensors and their advantages over conventional surface-sensitive transducers. The content includes the analysis of interface circuits, the investigation of viscoelasticity and micromechanics, as well as surface roughness in connection with QCM. Additionally, it covers analytical applications such as the detection of nucleic acids, immunosensors, and cell monitoring through impregnated polymers. The book also presents advanced derivatives of the classical QCM that enable new detection schemes by combining multiple techniques.