Although the basic physics of millimetre wave reflectometry has been known for many years, it is only recentlythat reliable measurements have been obtained with the technique in applications on tokamak plasmas. This is primarilybecause for these plasmas the electron density is a rapidly fluctuating parameter and these fluctuations generate broad-band'noise' in the measurement systems. Also, several important experimental requirements were not originally realised.Recently, special techniques and hardware for processing the signals have been developed, and appropriate care taken indesigning and implementing the measurement systems. As a result, reliable measurements are now being obtainedroutinely and are being used extensively in plasma physics studies.Reflectometry has the potential to provide measurements of the spatial dependence of the electron density, ie. theelectron density profile, during quasi-stationary periods of the plasma. It can also provide measurements of themovement of specific density layers during fast phenomena, ie. density transients. In principle, information on the broadband density fluctuations can also be obtained.Extensive use is made of reflectometry at JET where substantial systems have been developed. In this paper, wediscuss the principles of the technique and describe the instrumentation on JET. Results demonstrating the performanceof the reflectometers are presented.