This paper is concerned with the phenomenon of enhanced plasma-frequency radiation in Tokamaks which has attracted much attention recently. Experimental results indicate that the radiation is associated with high energy runaway electrons. However the emission mechanism has not been completely understood. In this paper we review and discuss two possible processes which can explain the observed enhanced radiation. One is an induced emission process which is due to an anomalous cyclotron resonance of the runaway electrons with waves. The other is a spontaneous radiation process mainly due to Cherenkov resonance. The former requires higher energies of the runaway electrons than that required in the latter. Thus the theory of the spontaneous emission is more appealing and deserve more attention. Extensive numerical calculations of the emissivity have been performed over a broad range of parameters. The discussion is also extended to the case in which the electron plasma frequency is higher than the cyclotron frequency, a situation may be reached by Alcator.