As a consequence of continuous growth in population, in standards of life and in industrial development, stress on water supplies is steadily increasing in the Mediterranean and other areas with water scarcity. Therefore, a need to use raw waters of lower quality has developed. In some of the countries of the Mediterranean and Middle East region, the governments have issued new large-scale programs to recover and reuse treated municipal waste waters, rehabilitate salinated and contaminated wells and desalinate brackish and other difficult raw waters from marginal sources. This paper presents trends for the above type of treatments and discusses alternatives and problems seen in implementing some of the possibilities. Benefits and problems encountered in "real-life" application of modern membrane technologies are presented in the example of three case histories on industrial size and pilot plants for reverse osmosis (RO) or nanofiltration (NF) treatment. Feed waters included are industrial, municipal and agricultural effluents as well as highly contaminated surface and well waters. For some of the cases, economic figures are also presented. Based on the experiences given in the case histories, conclusions for the selection of feed waters and for future technical concepts are drawn.