The fundamental principles of the quartz crystal microbalance (QCM) and its use in electrochemistry are explained. Based on the converse piezoelectric effect, these oscillating sensor devices are able to detect mass changes down to the nanogram range. Mechanical networks and electrical equivalent circuits are used to introduce Sauerbrey's equation. Moreover, model calculations explain the loading parameter R in terms of impedance analysis. Some basic circuits and methods are introduced including oscillator techniques, common impedance and dissipative analysis, and the fast relative impedance scanning (FIS-)QCM especially in association with common electrochemical methods. Some recent examples based on battery research show some success using this method.