High performance plasmas were obtained in many JET experiments, both before and after the major shut down of 1992-93. In this paper, a summary is presented, of the MHD activity which , in both the old and the new JET configuration, appears to be most clearly correlated to the collapse of the neutron yield. The paper will discuss aspects of MHD observations in three types of performance limitation; a) slow roll-overs of the neutron rate, which appear to start with the growth of low n modes in the outer 30% of the plasma, b) sawteeth coupled to EL Ms, and c) giant ELMs.