JET has made a strong contribution to the understanding of stability issues for the tokamak. An overview of its main achievements is presented, with emphasis on the latest progress in resolving the key issues for ITER. In particular, we conclude that control or avoidance strategies for neoclassical tearing modes (NTMs) will be necessary for good performance in ITER. JET studies have provided insights into the transport effects, seeding, underlying physics, and threshold scaling of NTMs. A range of mechanisms are found that can trigger performance-impacting NTMs with various mode numbers. Experiments have highlighted the key role of the sawtooth in triggering the NTM and have developed sawtooth control. The underlying physics suggests increased likelihood of NTM triggering as ITER scales are approached. Extensions have also been made in understanding of error field locked modes and resistive wall modes (RWMs). The predictions for ITER of error field locked mode thresholds have been developed and refined taking account of JET data. Direct inference from experimental studies and benchmarking of magnetohydrodynamic codes have both contributed to improved understanding of RWM stability in ITER. From these developments, and from the parameter space it accesses, JET continues to provide an essential role, and unique operating points, to further test and resolve the stability issues of tokamak physics.