The free-electron laser (FEL) offers the advantage of a potential large tunability since the FEL gain remains subtantially high throuhout the infrared spectral range and the reflectivity of metal mirrors used in the optical cavity remains close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator, the mixing of transverse modes when this chamber is used as a waveguide and its effects on the output coupling. At long wavelengths (THz range) these effects can inhibit laser tunability. We discuss these for various situations. At CLIO, we have obtained an FEL tunable from 3 to 150 µm by operating the accelerator between 50 and 12 MeV, except for a drop of power around 60 µm. Also a two colors mode is operated at wavelengths between 5 and 40 µm. Various type of applications are taking place at the CLIO FEL facility. We show some examples of these experiments.