The possibility of extracting energy in a controlled manner from the nuclei of deuterium holds out the hope of a permanent solution of the problem of the supply of energy. It involves heating the gas to temperatures in the 100-million degree range. The outstanding difficulty is to prevent the heat escaping. Inevitable loss by radiation can be accepted. Loss by conduction must be greatly reduced. This means using magnetic fields.Among the methods that have been tried to contain a plasma, three are: the pinch discharge in which a large current is passed through the rarified deuterium in a torus; containment by magnetic mirrors in which the diamagnetic character of the plasma is used to exclude it from regions of high magnetic field; Spitzer's stellerator in which gas is contained by re-entrant lines of force due to currents outside the vessel which holds it.Difficulties arise in all cases; for the pinch discharge and to some extent the stellerator, these are due to some of the many forms of instability which can affect plasma in magnetic fields ; for the magnetic mirrors to the poor containment at relatively low temperatures and to difficulties of injection. No method has yet been successful, but all these are very hopeful.