Composite membranes have multiple layers or parts of different separation ability in them ; composite membranes have been expected to have superior performance to homogeneous membranes. Most commonly used are composite membranes in series : asymmetric membranes in reverse osmosis and bipolar membranes in (electro) dialysis. In this review, first of all, transport models of homogeneous membranes, composite membranes in series and in parallel were outlined by following Kedem and Katchalsky's transport model. Second, models of composite membranes applied in reverse osmosis were explained. They have been used mainly to analyze transport resistance of supporting layers. Third, bipolar membranes, membranes that have two charged layers of opposite charge, were surveyed. They have been investigated in (electro) dialysis membranes in order to improve mono- and divalent ion selectivities. Finally, bipolar reverse osmosis membranes were discussed from both theoretical and experimental viewpoints. Bipolar reverse osmosis membranes were prepared by adsorption of polyelectrolyte on the surface of loose reverse osmosis membranes of negative charge. Ion selectivity of monovalent ions over divalent ones for both anions and cations was improved. Transport model of bipolar reverse osmosis membranes was also proposed.