The application of acoustic wave microsensors for mass sensing will be reviewed with focus on the quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices. The use of QCM and SAW devices in chemical sensing as well as in the determination of solid and liquid properties will be described. In chemical sensing, it is unlikely that a single sensor with a single coating will display a selective and reversible response to a given analyte in a mixture. Alternative strategies such as the use of sensor arrays and the use of sampling devices can be used to improve performance. QCM sensors (QCMs) will oscillate under liquids; their use in under-liquid sensing will be discussed.