Free Electron Lasers (FELs) are intense, monochromatic, tunable and coherent light sources that, thanks to their unique features, are opening new research opportunities in many different fields. For several years FEL's have been operating with success in the infrared; recently a considerable progress has been made in the UV region, where lasing wavelengths shorter than 190 nm have been able to be obtained, disclosing new opportunities for the study of the electronic properties of thin films and microstructures. UV FEL's are powerful instruments to probe the excited states of matter, especially when used in combination with synchrotron radiation in a pump-probe configuration. This kind of two-photon photoemission experiments, performed on Si surfaces and interfaces, have allowed a detailed study of the behavior of the non-equilibrium carrier distribution, making it possible to observe the role of interface states in the charge flow mechanism at photoexcited surfaces. Beside their use in combination with synchrotron radiation, UV FEL's have some unique features which make them particularly attractive also per se: in particular, being very intense and naturally tunable, they are ideal light sources for threshold photoelectron emission microscopy, and thus powerful instruments to study the electronic properties of nanostructures.