Recent advances in membrane technology have prompted the rapid growth of the Reverse Osmosis (RO) desalination in comparison to other water desalination technologies. One of the major problems of RO is fouling which leads to major reduction in the efficiency of this process. RO membranes are usually fouled with colloids, humic substances, micro-organisms, and heavy metals. This is why it is critical to treat the feed water prior to RO filtration. Conventional pre-treatment methods include processes such as coagulation, adsorption, sedimentation, flotation, sand filtration, disinfection, and the addition of anti-scalants. Recently, membrane pre-treatment processes including micro-filtration (MF), ultrafiltration (UF) and nanofiltration have been introduced prior to RO, with or as a replacement for conventional pre-treatment. These processes are useful in providing feed water superior in quality to conventional pre-treatment, but they are limited in the range of pollutant removal and operating conditions. Full description of water composition and the interactions and aggregations between the contaminants found in feed water for RO desalination is shown in this review. The review includes introduction to membranes, including retention and fouling mechanisms, conventional and membrane pre-treatment, and membrane backwashing. It also highlights the role of coagulation and adsorption in the pre-treatment process and the impact of integration of coagulation and/or adsorption with membrane pre-treatment.