The tokamak field configuration is based on plasma current flowing along a predominantly toroidal magnetic field. The joule dissipation associated with this current heats the plasma ions to typical temperatures of about 1 keV. Auxiliary heating by means of neutral atom beams has been used to raise the ion temperature to about 6 keV; high-frequency wave heating appears equally promising. The magnetohydrodynamic stability of the tokamak has been demonstrated experimentally for values of $\beta $ (ratio of mean plasma pressure to magnetic pressure) up to 3%, confirming theoretical expectations of stable reactor operation. Ion-energy confinement is found to be close to the optimal theoretical prediction. Electron-energy confinement is anomalous, but its observed scaling is compatible with a moderate-sized tokamak reactor. The principal remaining plasma physics problem is believed to be the control of `impurity' ions associated with wall interactions and with the burn-up of deuteriumtritium fuel.