In the research on controlled thermonuclear fusion there is a wide utilization of electromagnetic (EM) sources with regard to both plasma diagnostics and plasma heating. In this paper the analysis is focused on the applications to magnetic fusion lines, principally on tokamaks. Concerning the diagnostics, scattering of EM waves, which has the widest and most various applications of EM sources, is reviewed. The physical principles of Thomson and resonance fluorescence scattering are illustrated and the requirements of EM sources are outlined. Finally the most urgent needs are reported indicating possible and useful developments of FELs. Heating is then discussed and the three principal frequency ranges are indicated. The present and future principal projects of magnetic fusion are reported together with their EM power requirements and wave frequencies for plasma heating. It is suggested that a possible application of FELs is in the field of electron cyclotron heating.