The main limitation to the performance of the JET optimised shear discharges is due to MHD instabilities mostly in the form of a disruptive limit. The structure of the MHD mode observed as a precursor to the disruption as measured from soft x-ray and ECE diagnostics shows a global ideal MHD mode. The measured mode structure is in good agreement with the calculated mode structure of the pressure driven kink mode. The disruptions occur at relatively low normalised beta (1≤βN≤2) in good agreement with calculated ideal MHD stability limits for the n=1 pressure driven kink mode. These low limits are mainly due to the extreme peaking factor of the pressure profiles. Other MHD instabilities observed in the JET optimised shear discharges include, usually benign, chirping modes. These modes, which occur in bursts during which the frequency changes, have the same mode structure as the disruption precursor but are driven unstable by fast particles.