Twenty years ago the construction of JET began and five years later the first plasma pulses were obtained. A highly successful series of high temperature plasma experiments which addressed all important issues of fusion physics, such as energy confinement, plasma heating and plasma-wall interaction, laid the foundation for a Preliminary Tritium Experiment in 1991, a three stage programme of progressively more closed divertors from 1992 to 1999 and an extended experimental campaign in D-T in autumn 1997. The key physics results from JET are presented, emphasising the synergy between progress in physics and engineering and the impact of the results on improving physics understanding and predictive capability. The physics discussion concentrates on confinement and performance issues in the H-mode and optimised shear regimes, on the development of closed, highly radiating divertors and on the results from the recent broad-based series of D-T experiments. These results include records in fusion performance, the first clear demonstration of alpha particle heating, the testing of ICRF heating scenarios in D-T and the characterisation of the ELMy H-mode in D-T allowing more accurate predictions for the performance of ITER or any other Next Step device.