Free electron lasers are electron beam-based sources of coherent, high-power tunable radiation which can, in principle, be designed to operate at any wavelength ranging from mm waves to X-rays. In recent years, these devices have successfully been operated at THz and hard X-ray wavelengths where conventional sources of radiation are either not available or are not very efficient. This chapter briefly discusses the principle of operation and important subsystems of FELs, their special features, and the typical characteristics of some operational FELs. A review of the present status of activity in India and abroad is also discussed briefly.