A variety of materials have been used in JET for wall protection and high heat flux components. The machine initially operated with metallic walls, but the inner surface of the vessel (~200m2) is now covered to more than 50% with fine grain and carbon fibre reinforced graphite tiles. The remaining wall area is carbonized. This paper presents the materials behaviour in the presence of plasma; their influence on plasma properties; the conditioning methods employed; a discussion of future enhancements of inner wall components and the planned use of beryllium as an alternative to the present concept of an allgraphite machine. It is essential for the further developnent of fusion that the experience gained in JET is transferred to the next machine, which should produce a burning plasma on a scale canparable to a reactor. Such a proposed machine is a single null divertor tokamak with the following parameters: 3m plasma minor radius, 7.5m major radius, elongation ~2, aspect ratio ~2.5, toroidal field 4.5T, pulse duration 2000s and fusion power up to 5GW. The underlying physics for the choice of these parameters and the basic design is presented. Based on this concept and the experience with materials in JET, the lay-out of inner wall components, as well as possibilities for plasma exhaust and refuelling are discussed. As a consequence of this assessment, open questions with respect to the physics of the plasma edge and materials properties are highlighted.