Concentration polarization (CP) in membrane systems is understood as the phenomenon of the emergence of concentration gradients in a solution near the membrane surface, which is a result of the selective transfer of certain components of the solution through the membrane under the influence of transmembrane driving forces. CP accompanies all types of membrane processes. It affects transfer conditions and reduces the efficiency of separation processes: in most cases, there is a decrease in the overall transfer rate and an increase in energy consumption, as well as a loss of permselectivity. This review examines the general patterns and features of the CP phenomenon in the processes of electrodialysis, reverse osmosis, nanofiltration, ultrafiltration, pervaporation, as well as in membrane sensor systems and fuel cells. The fundamental principles of the CP phenomenon and experimental methods for its study are considered.