Recent theoretical results on MHD instabilities in heliotron/torsatron are reviewed. By comparing the results with experimental data in Heliotron E, Heliotron DR and ATF, it is pointed out that resistive interchange modes are the most crucial instabilities, since the magnetic hill occupies a substantial region of the plasma column.
Development of three-dimensional MHD equilibrium codes has made significant progress. By applying the local stability criteria shown by D(I) (ideal MHD mode) and D(R) (resistive MHD mode) to the equilibria given by the three-dimensional codes such as BETA and VMEC, stability thresholds for the low n ideal modes or the low n resistive modes may be estimated with reasonable accuracy, where n is a toroidal mode number.