This paper describes the Joint European Torus JET device which was built as a European collaboration effort, with the aim of testing the scientific feasibility of producing controlled thermonuclear reactions between light nuclei with a net yield of energy. JET is the largest magnetic confinement machine in the world both in physical size and in the magnitude of the plasma current 5xl06 Amperes. The machine carne into operation in mid 1983 and has followed the first stages of a planned evolution, in which the performance is progressively increased mainly by adding more heating power and which willculminate in eventual operation in a deuterium tritium 'This will permit study of the plasma performance when there is a substantial power input from the a particle So far operating in deuterium gas with BMW of additional heating by neutral beams, a peak ion temperature of 12keV has been obtained with a corresponding fusion product density 18x1018 m­3s. If the same conditions were to be achieved in a deuterium tritium of thermonuclear power output to the heating power input, Q, would be ~0.1. It is expected that following further technical improvements to JET, "scientific breakthrough" (namely Q = 1) will be achieved. The next step after JET will be to study a burning or ignited plasma in which no power input is required because energy losses are balanced by a particle The requirements for such an experiment will become increasingly clear as more data is obtained frcrn JET. At present it seems likely that a larger apparatus will be needed with a plasma current capability of 12­15MA. These requirements for the thermonuclear furnace remain broadly consistent with the known technological constraints on an eventual power reactor.