AbstractJET has accumulated practical experience of plasma operation in both limiter and divertor configurations using massive beryllium plasma-facing components. These latter have shown evidence, even after a very short operational phase, for localised melting of the beryllium surface. Sustained melting results in substantial transport of the liquid beryllium metal; there is, however, no evidence for low cycle termal fatigue under this extreme environment. Design has proceeded by thermal testing of prototypes with analysis used for interpretation only. For flux densities ≤5 MW m−2, T surface ≤1000°C, typically 200 pulses of 6–8 s can be sustained with plastic deformation and no surface fatigue cracking. The latter is seen for more extreme conditions, i.e. flux densities in excess of 14 MW m−2, pulse lengths ≤0.5 s surface temperature >700°C. Failure of thin cladding of beryllium to heat sinks, which tends to occur along the beryllium to substrate interface, was insufficient to produce fatigue effects in the bulk beryllium material, at fluxes ≤10 MW m−2.