The name marfe is given to the toroidally symmetric, but poloidally asymmetric, band of impurity radiation observed in tokamaks. This phenomenon is the result of an instability which arises when a local change in the radiated power, brought about by a temperature perturbation, is greater that the compensatory heat flow induced by the temperature change. The instability takes place in the outer region of the plasma where the temperature is low and the impurity radiation is correspondingly high. Stability depends not only on the temperature but also on the temperature profile in the region where it is formed. The straightforward way of solving the problem is first to obtain an equilibrium solution for the temperature in the presence of the radiation for each particular case, and then to calculate its stability. We shall here use a method which combines these processes and leads to a general but simple stability diagram.