Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with ICRH accelerated injected ⁴He beam ions provide a new tool for fast particle and magnetohydrodynamic studies, with up to 80–90% of plasma heating by fast ⁴He ions. With or without impurity seeding, a quasi-steady-state high confinement (H₉₈ = 1), high density (n_e/n_GW = 0.9–1) and high β (β_N = 2) ELMy H-mode has been achieved by operating near the ITER triangularity (δ~ 0.40–0.5) and safety factor (q₉₅~ 3), at Z_eff~ 1.5–2. In advanced tokamak (AT) scenarios, internal transport barriers (ITBs) are now characterized in real time with a new criterion, . Tailoring of the current profile with lower hybrid current drive provides reliable access to a variety of q profiles, lowering access power for barrier formation. Rational q surfaces appear to be associated with ITB formation. Alfvén cascades were observed in reversed shear plasmas, providing identification of q profile evolution. Plasmas with 'current holes' were observed and modelled. Transient high confinement AT regimes with H₈₉ = 3.3, β_N = 2.4 and ITER-relevant q < 5 were achieved with reversed magnetic shear. Quasi-stationary ITBs are developed with full non-inductive current drive, including ~50% bootstrap current. A record duration of ITBs was achieved, up to 11 s, approaching the resistive time. For the first time, pressure and current profiles of AT regimes are controlled by a real-time feedback system, in separate experiments. Erosion and co-deposition studies with a quartz micro-balance show reduced co-deposition. Measured divertor thermal loads during disruptions in JET could modify ITER assumptions.