The purpose of this chapter is to provide an overview of the different types of instrumentation used in process plants. The chapter focuses primarily on flow rate, level, temperature and pressure instruments. Although density, concentration and pH measurements, among others, are sometimes performed on-line, they are more often made in the laboratory. However, some on-line density measurements are discussed. It is also customary to include valves as part of instrumentation, but because the book does not discuss valve selection and location, their analysis is omitted. Finally, a brief discussion on new emerging tendencies in signal processing is added. A complete survey of instrumentation can be found in the handbooks edited by Liptak (1997) and Webster (1999).