In JET both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an mhd unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first there is a loss of energy from the central region and in the second there is a more rapid fall to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the mhd instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qy = 2. These disruptions do not have extended precursors and minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit disruption.