When the objectives and the parameters of JET were defined about ten years ago, the existing knowledge of plasma behaviour in tokamaks showed the advantages of the axial symmetry of magnetic configuration, as well as the leading role of the plasma current in heat confinement. After two years of operation, JET has proved to be the world's most successful device in thermonuclear fusion research. Whereas achievement of the final aim (i.e. the confinement of plasma with temperature and density such that thermonuclear power dominates the heat losses) has never been nearer, results show that plasma behaviour is even more complex than initially anticipated and nonlinear physics, leading to turbulence or chaos, is involved. How this is likely to modify the JET programme is addressed, as well as the prospects for a future reactor.