After difficult times in the nineties, Reverse Osmosis (RO) desalination nowadays gains again acceptance around the world and plays a larger and more important role in the municipal desalination market for small, medium and large scale applications. Since RO desalination plants depend on high quality feed water to ensure reliable and stable operation of the RO system, pretreatment with UltraFiltration (UF) membranes must be given strong consideration. UF membranes, having been proven in a wide range of much more difficult liquid environments such as municipal and industrial wastewaters, have become the preferred pretreatment solution. UF pretreatment also allows the economical utilization of RO membranes in areas where membrane desalination has not been considered as the appropriate technology due to difficult raw water conditions. Furthermore, using UF membrane technology to produce high quality pretreated feedwater allows troublesome RO installations to be brought to design productivity levels. UF., unlike conventional pretreatment technologies, provides a physical barrier to particulate and colloidal material and ensures that RO plants can operate on a continuous basis, at high and stable fluxes, at higher recovery rates, and also allow a better control of Boron limit values. All of these allows an increase in RO productivity and a decrease in life time costs.