FERMI is a single-pass FEL user-facility designed to cover the wavelength range from 100 nm to 4 nm. It uses a linear accelerator to generate a beam of relativistic electrons that is made to propagate through a long series of undulators. A conventional laser superposed to the electron beam in the first undulator (seeding) is utilized to induce coherent emission, which is exponentially amplified when the electrons propagate through the following undulator magnetic field. Among existing FEL sources FERMI is unique in many respects. The implemented seeded FEL scheme guarantees the generation of high transversely and longitudinally coherent pulses and the use of APPLE II-type undulators allows for variable polarization, from completely linear both horizontally and vertically to circular in both directions.