Concepts and historical review of free electron laser are reported. Since the experiment at Stanford University was successful in 1977, great numberof FEL experiments with various accelerators were successfully performed to observe amplification and oscillation of radiations in the wavelength range of 0.5 μm-103μm. As for shorter wavelength FELs, various ideas have been proposed and some experimental facilities in XUV and soft X-ray spectral rangewill be completed near future to begin oscillation experiments.