Desalination is an extremely effective too] for converting seawater, brackish water or wastewater into fresh water for usage. However, a pre-treatment is necessary to ensure that feed water will not cause clogging problems and precipitation on the reverse osmosis membrane. Pre-treatment of seawater feeding reverse osmosis (RO) membranes is recognised as a key in designing desalination plants. This paper covers the experience of Veolia Water through several examples of desalination plants at main sites around the world. This paper shows that the pre-treatment process must to be adapted to the seawater quality to be treated and to ensure the best quality upstream of the membranes. Depending on several parameters which influence the choice of the pre-treatment like dissolved organic carbon, SDI, turbidity, algae content and their evolution during the seasons, temperature, etc., the pretreatment can comprise different technologies: conventional pre-treatment (ballasted sedimentation, air flotation, dual media filtration, mono media filtration, double stage filtration, etc.) or advanced technologies including membranes coupled with a conventional process. This paper presents the approach of Veolia Water Solutions & Technologies for the desalination plants through the application of different technologies used for the brackish waters (groundwater and estuary), the seawaters (Mediterranean Sea, Gulf of Oman, Persian Gulf) and the wastewater reuse. The examples presented in this paper demonstrate that maximum benefit is attained by the use of an adapted pre-treatment and the scientific approach developed by Veolia to minimize the fouling problems allows a good protection of the membranes and assures a long life time.