The number and size of membrane based plants for the production of potable water and for the treatment of wastewater are increasing rapidly on our planet. The processes involved are reverse osmosis/nanofiltration and ultrafiltration/microfiltration. These plants produce clean water within one of the following scenarios: (1) Feed source seawater, supply unlimited, product potable water, (2) feed source brackish or human-contaminated well water, supply limited, product potable water; (3) feed source polluted water from rivers or lakes, supply almost unlimited, product potable water; and (4) feed source waste water, product high quality clean water for farming/gardening or for discharge into rivers/lakes. For these scenarios examples are presented which call be considered as 'advanced'. Based on these examples the potential and the limits of the processes are discussed taking into account availability, safety with respect to product quality and costs.