It is a well established theory that the success of a membrane based desalination system depends to a large extent on how effective the pretreatment process is. Membrane-based plants demand a stringent pretreatment technology. Ultrafiltration as a pretreatment offers a number of benefits to the reverse osmosis process by way of increased flux, increase in plant recovery, RO membrane life, decreased cleaning frequency, decreased chemical and energy consumption. Quite a good number of mega-sized plants have gone for UF pretreatment after successful trial runs. Piloting of the conditions with extremities of the raw water chemistry is advocated. Threadbare analysis of the results in terms of filter effluent turbidity, fouling index and temperature effects at some facilities is presented. A number of RO desalination plants have implemented UF pretreatment to safeguard the downstream equipment and enhance the plant availability. Pretreatment technology has not kept pace with the advances in the membranes such as fouling resistant (FR), extra low energy (XLE). The filtration efficiency of conventional pretreatment in treating aggressive raw water is vividly brought out with comparisons and shortcomings encountered. Some of the specific advantages of the membrane systems are less operator attention, produces lower filtrate SDI values and do not need source water conditioning.