The isotope dependence of plasma transport has a significant impact on the performance of future D-T experiments in JET and ITER and eventually on the fusion gain and economics of future reactors. In preparation for future D-T operation on JET, dedicated experiments and comprehensive transport analysis were performed in H, D and H-D mixed plasmas. The analysis of the data has demonstrated an unexpectedly strong and favourable dependence of the global confinement of energy, momentum and particles in ELMy H-mode plasmas on the atomic mass of the main ion species, scaling as τE~A0.5 [C.F. Maggi 2018, JET Team 1999], i.e. opposite to the expectations based only on local gyro-Bohm (GB) scaling, τE~A‒0.5, and stronger than in the commonly used Hmode scaling for the energy confinement [G. Saibene 1999, ITER Physics Basis 1999]. The scaling of momentum transport and particle confinement with isotope mass is very similar to that of energy transport. Nonliner local GENE gyrokinetic analysis shows that the observed anti-GB heat fluxes are accounted for if collisions, E×B shear and plasma dilution with low-Z impurities (9Be) are included in the analysis. For L-mode plasmas a weaker positive isotope scaling τE~A0.14 has been found in JET [C.F. Maggi 2018], similar to ITER97-L scaling [S. M. Kaye 1997]. Flux-driven quasi-linear gyrofluid calculations using JETTO-TGLF in L-mode show that local GB scaling is overruled when stiff transport (as is the case for ITG’s) is combined with an imposed boundary condition taken from the experiment, in this case predicting no isotope dependence. Dimensionless isotope identity experiments in JET L-mode plasmas demostrated scale invariance, confirming that core transport physics is governed as expected by 4 dimensionless parameters (ρ*, nu*, β, q), consistently with global quasi-linear gyrokinetic TGLF calculations [Maggi 2019]. We compare findings in JET with those in different devices and discuss the possible reasons for the different isotope scalings reported from different devices. The diversity of 0bservations suggests that the differences may result not only from differences affecting the core, e.g. heating schemes, but are to a large part be due to differences in device-specific edge and wall conditions, pointing to the urgency of better understanding and controlling pedestal and edge processes.