This lecture reviews the progress made towards releasing energy from the fusion of thelight nuclei. To achieve this it is necessary to create an extremely hot plasma containing the light nuclei and to hold this plasma together long enough to obtain a net surplus of energy. The only promising way of confining such a hot plasma for sufficient times and isolating it from material surroundings is by means of magnetic fields. Unfortunately plasmas have shown themselves capable of escaping through magenetic fields by a number of forms of instability. The principles of magnetic confinement of plasmas are discussed and the main types of magnetic traps described.