The Stanford Research model QCM200 Quartz Crystal Microbalance measures mass and viscosity in processes occurring at or near surfaces, or within thin films. This system includes a controller, crystal oscillator electronics, crystal holder, three quartz crystals, and Windows / Mac software. The instrument reads the resonant frequency and resistance of a 5 MHz, AT-cut quartz crystal. The resonant frequency changes as a linear function of the mass of material deposited on the crystal surface. The resistance at resonance changes with the viscosity / elasticity of the material (film or liquid) in contact with the crystal surface. As a gravimetric instrument, the QCM200 can measure mass ranging from micrograms to fractions of a nanogram. Detection limits correspond to submonolayers of atoms. Observations of conformational changes, such as phase transitions, swelling, and cross-linking, can easily be made.