A range of membrane processes are used to separate ne particles and colloids, macromolecules such as proteins, lowmolecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microltration (MF), ultraltration (UF), nanoltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong inuence on membrane performance. For example, for the pressuredriven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact.