Since 2016, the JET scientific programme is engaged in a multi-campaign effort including experiments in D, H and T [1], leading to 2020 and the first experiments with 50%/50% D–T mixtures since 1997 (DTE1 campaign [2, 3]), where 16 MW of fusion power was achieved transiently and 4 MW in the steady state, and the first ever D–T plasmas with the ITER mix of plasma-facing component materials [4–6]. This effort is also driven by the EUROfusion research roadmap to secure the success of the future operation of ITER via specific preparation and experiments, including D–T operation of JET [7]. For this purpose, a concerted physics and technology programme was launched with a view to prepare the second JET D–T campaign (DTE2)[8]. This overview paper addresses the key elements developed by the JET programme directly contributing to the D–T preparation. JET is a unique device in the sense that it has been designed from the start as a D–T fusion tokamak with the aim to study plasma behavior in conditions and dimensions approaching those required in a fusion reactor, and therefore it has the capability to study the physics of alpha power. JET is equipped with a tritium plant and is capable of efficiently confining the alpha particles in the plasma (90% of alphas confined for plasma current above 2.5 MA) thanks to its size and the plasma current it can reach (up to 5 MA in the present configuration).In addition, since DTE1 in 1997, the original carbon wall of JET has been changed to an ITER-like wall with a tungsten divertor and a beryllium first wall with the total input power upgraded to 40 MW and the set of diagnostics dramatically …