AbstractThe Free Electron Laser (FEL) has reached a stage of development, in which serious consideration of its potential use in fusion research becomes obvious.The principles of operation of the FEL are discussed, together with the similarities and differences with classical devices like gyrotrons on the one hand, and ordinary lasers on the other hand. The FEL's potentially high efficiency and large unit size, combined with wide tunability, are among the outstanding characteristics. It is to be expected that operation in the ‘optical guiding’ regime, for which criteria will be given, is the route to enhanced performance of the devices.An overview is presented of current projects for FEL development, either in direct connection with, or possibly relevant for, fusion. Applications in this area include both heating, profile control and diagnostics in magnetic fusion, and driving implosions in inertial confinement.