Because of its large size, single null divertor, and exible magnetic geometry, JET is capable of producing the most reactor-relevant plasmas of any present generation tokamak. In the recent deuterium-tritium experiments the fusion performance of these plasmas was tested for the rst time. Over 4MW of fusion power was produced in a high power, steady state pulse of 5s, limited by the duration of the heating power. The fusion QE , dened simply as the fusion energy produced divided by the input energy over this 5s interval, was 0.18. The performance of our DT ELMy H-mode discharges extrapolates to ignition in ITER and thus provides increased condence in its current design. Operation at low q95 is possible in JET with no degradation in connement and provides an improved margin to ignition when extrapolated to ITER.