The design of the Joint European Torus was conceived with inherent flexibility to accommodate modifications and upgradings to match the evolving requirements of the physics programme, while preserving basic machine structure. The first major upgrading was to increase the plasma current capability from 4.8 to 7MA in limiter configuration and from 3.0 to 5MA in X-point configuration. The second change was the progressive covering of the vessel walls with low-Z materials such as graphite and beryllium. The most recent major modification was to make JET into a pumped divertor machine. Three divertors are being tested in sequence (Mark I, II, IIGB), in support of the ITER design. JET is operating at present with Mark II both in D-D and in D-T. Thus, the installation of Mark IIGB will be performed using only remote handling techniques. Divertor plasmas are more vertically unstable, and so a new plasma control system had to be designed and implemented. The engineering instrumentation of the machine has been upgraded, for machine protection and to monitor and study new phenomena such as sideways vessel displacements, caused by plasma disruptions. An in depth reassessment of the toroidal coils, of the mechanical structure and of the vessel is in progress. This includes finite element calculations and mechanical tests on samples and on two toroidal field whole coils, to evaluate the machine capability to operate at higher toroidal field (from 3.4T to 4.0T) and operation at 3.8T has been undertaken already. In the early phase of the 1997 D-T campaign ~13MW of fusion power have been produced.