The JET design, which started in 1973, introduced bold new concepts such as D-shaped plasmas in large tokamaks, a closed-loop tritium plant, and the use of beryllium as a first-wall material. It implied increasing by two orders of magnitudes the plasma volume and the heating power compared to the standard at the time. During the JET Joint Undertaking operation from 1978 to 1999, most of these design parameters were exceeded. After achieving all of its initial objectives, JET was upgraded and modified to establish H-mode scaling and to perform comprehensive studies of divertor and advanced tokamak concepts. JET holds all records in fusion power and energy and has allowed a unique experience in D-T operation to be gained. The JET results have made a decisive contribution to the scaling laws on which the basic layout and the dimensions of ITER are based. JET today under its new EFDA-JET organization is still the most powerful fusion device operating in the world, with potential to extend its performance even further. It has the essential mission to prepare for D-T burn in ITER and to train a new generation of scientists for developing fusion as an energy source.