[en] A review is presented of the theoretical and experimental results which have been obtained to assess the potential of a Free Electron Laser (FEL) and to estimate the electron beam requirements with regard to energy, current and emittance. It is shown that with a FEL it is possible to obtain a radiation source which can be continuously tuned over a wide range from the far infra-red to the U-V, by varying the electron beam energy over a range approximately 1-250 MeV. The electron beam is required to be of high quality with very small energy spread and small transverse emittance. With presently developed electron beam sources of suitable quality, the conversion efficiency limits the output power to a mean of a few watts and a peak of the order of tens of kilo-watts. To obtain high output power requires the development of high current electron beam sources with high beam quality. The required beam quality is dependent on the magnet design and so further development is also required of high periodicity transverse field magnets with uniform transverse field across the magnet aperture. The spread in the electron distribution resulting from the dynamics in the FEL would appear to prevent any advantage being gained by recycling the electron beam. (U.K.)