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VIRGO is a recycled Michelson interferometer where each arm is replacedby a 3 km long Fabry-Perot cavity. It is designed to detect gravitational waves emitted by astrophysical sources. Mirrors are suspended in vacuumby high performance suspensions, so that detection is possibly starting from very lowfrequencies, a few Hz, up to a few kHz.The work presented in this thesis is focused on the longitudinal control of VIRGO,that is the control of the position of the test masses along the light beam direction.Relative displacements of the mirrors need to be actively controlled in order to bringand keep the interferometer on its working point. By following the evolution of theVIRGO commissioning, suitable strategies have been designed and applied to differ-ent optical confgurations: a single Fabry-Perot cavity, a Michelson interferometerwith Fabry-Perot cavities in the arms (the recombined ITF), and finally the fullinterferometer (the recycled ITF). Once the lock of full VIRGO was achieved, theprocess of investigating and reducing the longitudinal control noises coupled intothe dark fringe signal began.