Reverse osmosis is a membrane process that utilizes the osmotic pressure difference by the applied force to carry out separation in a manner opposite to natural osmosis. Reverse osmosis technology involves two different material streams: dilute permeate and concentrate retentate. Three different transport models (nonporous membrane model, porous membrane model, and irreversible thermodynamic model) can be used to predict the performance of reverse osmosis. Membrane fouling, including colloidal fouling, biofouling, and organic and inorganic fouling, and specifically inorganic fouling and the relevant control measures, are discussed in Chapter 5. The design of a reverse osmosis system is discussed in terms of selection of membrane materials. The types of membrane and the general operating conditions are also specified. Recent reverse osmosis applications in both upstream and downstream bioprocessing are also covered. Current reverse osmosis technology is energy-intensive and inflexible, so advanced hybrid systems and fabrication of reverse osmosis membranes from new materials appear to be the solution for unlocking a greater future potential of reverse osmosis technology.