An important requirement for fusion experiments like JET is the accurate and reliable measurement of the plasma parameters. The most important quantities to be measured include the discharge current, the position and shape of the plasma boundary, the densities and temperatures of the ions and electrons, the concentrations of impurity ions and the various energy input and loss mechanisms. Because all of these quantities vary in space and time, a large number of measurement channels is needed, and a large amount of data is generated for each discharge. Measurements in high-temperature plasmas are difficult because material probes cannot be used except at the extreme edge of the discharge. For measurements in the hot central regions, non-material probes based on electromagnetic radiation or particle beams are used together with analysis of electromagnetic radiation and particles emitted spontaneously from the plasma. A wide variety of experimental techniques is utilized and the application of some of these methods to JET will be described.