This work presents a general study on Free Electron Lasers (FEL) in the seeded configuration. Three examples are given : the UVSOR-II FEL (Okazaki, Japan), the SPARC FEL (Frascati, Italy) and the ARC?EN?CIEL project FEL (France). In the case of the UVSOR?II FEL, seeded with a Ti :Sa laser at 1 kHz repetition rate, several studies have been performed : electron beam dynamics, spatial coherence, spectral structure and angular distribution of the radiation, optimization in helical mode. In the case of the SPARC FEL, the injection of a harmonic source generated in rare gaz (HHG) is foreseen. This original combination stands as an attractive source for users with a high temporal and spatial coherence degree together with a high intensity from UV to X rays. A dedicated harmonic source has been designed, assembled and tested for the SPARC FEL. The operation of the combined devices should start in Winter 2008, allowing fine caracterization of the HHG-FEL association and further demonstration of original HGHG FELs configuration. Finally, during the simulation studies performed for the design of the ARC?EN?CIEL light sources, a new propagation regime of the FEL pulse has been observed and is still under study.