The Joint European Torus (JET) has demonstrated its ability to produce plasma with parameters (e.g. density, temperature and confinement time) closer to self-sustaining, thermonuclear conditions than any other tokamak. The increased energy confinement times (to greater than 1s) which have been obtained justify the extrapolation of tokamaks in size to JET. As a result of the investigation of new plasma parameter regimes, considerable knowledge has been obtained concerning plasma energy and particle transport, and methods have been found to reduce the problem of impurities and to provide refueling. Although there is an improved knowledge of high-temperature plasma behaviour, understanding is not yet complete. "Classical" theory is found to provide agreement with experiment in several situations. The degree of anomaly in other cases has been quantitatively examined by a variety of methods but does not appear to present an insuperable barrier to the achievement of reactor-relevant plasma conditions. Overall, the JET experimental results are very encouraging for the success of this research and they are tantalisingly close to fulfilling the original objective for this experiment.