In this paper the principles, potential and limits of reverse osmosis, nanofiltration and ultrafiltration are discussed, using new applications in the area of waste water treatment as examples. The examples emphasize, among others, two features which are essential for membrane processes in general: the purely physical nature of the separation principle, and the modular design of membrane processes. These are the features which make membrane processes potential candidates for a process integrated effluent treatment, not only aimed at clarification of the water but at the recovery of the dissolved/suspended components as well. We believe that the potential of the presently available membranes is not yet fully explored. Modules and processes can and will be further developed with the general task of improving reliability and, simultaneously, lowering the operation and investment costs.