The authors discuss two aspects of the theory of radio frequency heating of tokamaks. The first is the development of a simplified mathematical treatment of ion cyclotron absorption and its application to a study of the effect of a small concentration of high energy fusion products. The second is an investigation of the interaction of high amplitude electron cyclotron waves with a plasma. The nonlinear behaviour of electrons in such a wave is studied, with a view to improving on the usual quasilinear diffusion term used to describe wave-particle interactions.