The growth of radio and other electromagnetic waves is considered in terms of the ion motions of Bailey's electromagneto-ionic theory, of a double electron stream and other factors. These conclusions are reached: (i) The electromagneto-ionic theory predicts spurious growing waves which do not correspond to any interchange of energy between gas and field but merely to the movements of the observer and emitter relative to the gas particles. It probably does not predict any real growing waves.(ii) The twelve different oscillatory modes inferred from the full dispersion equation correspond to four unreal waves and four pairs of real waves: two pairs of hydromagnetic waves (becoming radio waves as a limiting high-frequency case), one pair of modified sound waves and one pair of modified electron-sound waves.(iii) Electromagnetic wave growth may possibly result from one and only one process: the trapping of ions between potential troughs of the space-charge wave which is an integral part of some electromagnetic waves. After trapping, the ions give kinetic energy to the wave by one of two known processes.(iv) Ion drift motions may introduce important effects not indicated by the wave equations.