Energy ranges and time scales for excitation and relaxation of intramolecular processes are discussed and compared with the operational characteristics of laser and synchrotron radiation sources. The basic physics of synchrotron radiation and undulator emission is presented. It is shown how undulators can be used to generate short wavelength harmonics. Free electron lasers of the Compton and Raman scattering types and the associated electron beam sources are described. The properties and applications of free electron lasers are reviewed.
