JET has a fully metallic, completely welded vacuum vessel which is bakeable up to 500oC. The internal surface is 1000m2, the volume 200m3, and the installed pumping speed for hydrogen ≈8500ls-1. The base pressure (mainly hydrogen) is typically smaller than 2x10-7 mbar at the operating temperature of 300oC. The total pressure for impurities can be as low as 5x10-10 mbar depending on conditioning methods applied and the wall materials used. The operation of JET showed that it was necessary to protect the metallic walls of the vessel against damage by energetic electrons. Initially fine grain, high purity graphite was used, later beryllium layers of a total thickness of about 1μm were evaporated onto the graphite and then massive beryllium was employed as the material in contact with the plasma. In this paper we shortly describe the principle of tokamak experiments, discuss the role of plasma contamination by impurities generated at the walls of the vacuum vessel, the design of the JET vacuum vessel, the wall conditioning techniques employed and the experience gained with the different wall