Surface acoustic waves (SAWs) excited on the surface of piezoelectric crystals form the basis of a family of microsensors that is sensitive, portable, cheap, and small. These microsensors can be used in a wide variety of applications to sense low levels of gases, physical quantities such as mass, temperature, pressure, flow and humidity, biological processes, corrosion, ions and particulate matter, and a variety of mechanical and electrical quantities. In contrast to other microsensor technologies, SAW microsensors can sensitively detect both electrical and mechanical phenomena. This is due to the fact that the SAW has both mechanical and electrical fields. In those applications requiring sensitivity, such as gas sensing and biosensing, the selectivity is provided by a receptor film applied to the piezoelectric substrate. In this paper different types of SAWs are described and prototype sensing geometries are presented. Particular attention is devoted to one of the most common SAW microsensor applications, namely, gas sensing. Finally, results obtained using SAW microsensors in a variety of applications are presented and discussed.