The Middle East region is regarded as the cradle of desalting, having many of the world＇s largest membrane systems in addition to extensive thermal distillation plants. Many important design and operation techniques have been developed and perfected in the region because of the need to solve problems locally. Efficient pretreatment design and operation is essential for the optimisation of RO plant performance irrespective of whether the feed source is natural or waste water. Poorly designed pretreatment and inadequate plant maintenance has often resulted in membrane failure and tarnished the reputation of the reverse osmosis process. This paper examines the importance of monitoring pretreatment plant used to control microbiological and general fouling and inhibiting the formation of inorganic scales. The paper gives examples of systems which have experienced problems and the technical recommendations made to improve pretreatment so as to improve membrane performance and operating and costs.