Membrane technologies are physical separation techniques that are becoming increasingly important in separation and purification processes. In water treatment, the driving force is most often a pressure gradient. Several technologies can be classified according to the size range of the membrane pores, enabling the retention of different species ranging from particles and microorganisms a few microns in size to molecules and ions at the nanoscale. This presentation will provide information on two related technologies—reverse osmosis and nanofiltration— to help decision-makers choose the most appropriate technique for their specific application. To address this question, we will highlight the differences in material structure, the transport mechanisms involved, and their implications for selectivity, productivity, and energy consumption. Examples of applications in the areas of drinking water treatment, desalination, and wastewater purification will be presented.