Water type is the cornerstone in reverse osmosis system designing. However, the systematic differentiation and identification of different natural waters are almost missing. In the RO industry all natural waters are usually grouped under two types: brackish (1000-15000 mg/l) and seawater(> 15,000 mg/l) as based on their total salinity. Only two RO membrane types are commercially available (i.e., BWRO and SWRO), which are currently used for all waters regardless of the wide variation in their chemical nature, even those of similar salinity. Most of the recommended pretreatment methods given by the membrane manufacturers are normally based on a single water type, either brackish or seawater. In addition, the current use of the misleading saturation indices (LSI, S&DSI, etc.) that developed originally for heating boilers and exchangers, led to serious inorganic scaling problems. These generalized approaches have serious impact on the performance and economics of RO desalination. The cost of $2-3 per 1m(3) of desalinated water is not surprising anymore, specially in the case of small-capacity plants. The present work concerns the critical importance of differentiation of natural waters in a systematic chemical classification that may provide assistance in RO system design, selection of membrane and proper pretreatment as well. From the experience gained in hydrochemistry and water analysis along the last 30 years, besides the extensive literature survey and the statistical analysis of more than 200 water analyses, the present authors were able to conclude a systematic water classification. The proposed classification is based on the earlier work of El-Manharawy and Hafez as well as the real molar-concentration of the dissolved ion associations of the investigated water samples which covered a wide range of natural surface and ground water types. It was possible to identify four major water classes (< 10, 150, 400 and > 600 mM chloride ion), including 10 subclasses of different water types [<0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 15, 20, > 20 (SO4/HCO3) molar ratio]. These classes and types cover the whole spectrum from salinity of similar to 200 mg/l up to similar to 60,000 mg/l. More detailed subdivisions could be derived when necessary. In light of the obtained water molar classification, it was possible to set guidelines for inorganic scale prediction and the suitable chemical pretreatment for specifically each of the proposed water types. Technical recommendations for RO system design and membrane selection are also provided in detail.