The ITER-like wall project in JET aims at an optimal use of the unique features of JET, such as beryllium and tritium compatibility, to explore operation within the limits of the ITER wall materials. A full replacement of the presently carbon based first wall will result in a mainly beryllium main chamber and a tungsten divertor. At the same time, the JET auxiliary heating power will be upgraded allowing access to ITER-relevant energy loss densities in disruptions and edge localised modes. In this way, the JET wall will go from being almost indestructible, to making the material driven operational constraints predicted for ITER a more immediate reality for JET. This paper describes the methodology being used to define and apply these limits whilst trying to maintain experimental flexibility.