The quartz crystal microbalance (QCM) is turning into an attractive tool for gravimetric measurements and applications and can be found in many research fields such as acoustic sensors (Chap. 2), chemical sensors (Chap. 9) and biosensors (Chap. 10). In general, QCM resonators are covered with noble metals such as gold or silver, usually by evaporation or sputtering, and can be used directly as electrodes or undergo further surface modification. Using such strategies, the study of processes ranging from electroplating to DNA immobilization becomes possible. In this chapter, two separate sequential preparation steps should be distinguished. Metallic deposition (acting as the ultrasonic wave generator) must be first considered as a support for modification. In the second step, organic or biochemical modifications are carried out before testing the modified QCM.