A membrane is a thin, film-like structure that separates two fluids. It acts as a selective barrier, allowing some particles or chemicals to pass through, but not others. In some cases, especially in anatomy, membrane may refer to a thin film that is primarily a separating structure rather than a selective barrier.
The concept of a membrane has been known since the eighteenth century, but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with microfiltration and ultra-filtration technologies. Since the 1980’s, these separation processes, along with electrodialysis, are employed in large plants and, today, a number of experienced companies serve the market.
A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream.
Membranes can be of various thickness, with homogeneous or heterogeneous structure. Membrane can also be classified according to their pore diameter. According to IUPAC, there are three different types of pore size classifications: microporous (dp < 2 nm), mesoporous (2 nm < dp < 50 nm) and macroporous (dp > 50 nm). Membranes can be neutral or charged, and particles transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into synthetic membranes and biological membranes.
Industrial use 
Certain features of membranes are responsible for the interest in using them as substitutes to consolidated industrial separation processes, like distillation, adsorption or extraction. Some advantages noted include:
- Less energy-intensive, since they do not require major phase changes
- Do not demand adsorbents or solvents, which may be expensive or difficult to handle
- Equipment simplicity and modularity, which facilitates the incorporation of more efficient membranes
- "Membranes on Polyolefins Plants Vent Recovery, Improvement Economics Program". by Intratec, ISBN 978-0615678917, Q3 2012.
- Zydney, Andrew L.; Zeman, Leos J. (1996). Microfiltration and ultrafiltration: principles and applications. New York: CRC. ISBN 0-8247-9735-3.
- Macroporous Materials Containing Three Dimensional Periodic Structures
- Mulder, Marcel (1996). Basic principles of membrane technology (2 ed.). Kluwer Academic: Springer. ISBN 0-7923-4248-8.