High fusion power experiments using D-T mixtures in ELM-free H-mode and optimised shear regimes in JET are reported. A fusion power of 16.1 MW has been produced in an ELM-free H-mode at 4.2MA/3.6T. The transient value of the fusion amplification factor was 0.95±0.17, consistent with the high value of nDT(0) τEdiaTi(0)=8.7x1020 m-3 s keV±20%, and maintained for about half an energy confinement time until excessive edge pressure gradients resulted in discharge termination by MHD instabilities. The ratio of D-D and D-T fusion powers (from separate but otherwise similar discharges) showed the expected factor of 210, validating D-D projections of D-T performance for similar pressure profiles and good plasma mixture control which was achieved by loading the vessel walls with the appropriate D-T mix. Magnetic fluctuation spectra showed no evidence of Alfvénic instabilities driven by alpha particles, in agreement with theoretical model calculations. Alpha particle heating has been unambiguously observed, its effect being separated successfully from possible isotope effects on energy confinement by varying the tritium concentration in otherwise similar discharges. The scan showed that there was no or at most a very weak isotope effect on the energy confinement time. The highest electron temperature was clearly correlated with the maximum alpha particle heating power and the optimum D-T mixture; the maximum increase was 1.3±0.23 keV with 1.3MW of alpha particle heating power, consistent with classical expectations for alpha particle confinement and heating. In the optimised shear regime, clear internal transport barriers were established for the first time in D-T, with a power similar to that required in D-D. The ion thermal conductivity in the plasma core approached neoclassical levels. Real time power control maintained the plasma core close to limits set by pressure gradient driven MHD instabilities, allowing 8.2MW of D-T fusion power with nDT(0) τEdia Ti(0) ≈ 1021 m-3 s keV, even though full optimisation was not possible within the imposed neutron budget. In addition, quasi steady-state discharges with simultaneous internal and edge transport barriers have been produced with high confinement and a fusion power of up to 7 MW; these double barrier discharges show a great potential for steadystate operation.