reverse osmosis is an increasingly popular though rather controversial winemaking intervention or manipulation based on the principle of cross-flow or tangential membrane filtration. In standard filtration procedures, the liquid flows perpendicularly to the filter surface, making clogging a major problem. In cross-flow filtration, the liquid flows parallel to the filter membrane so that it helps scour the surface and prevent clogging. The liquid flows at pressure, and it is this pressure that causes water and some salts to pass through the membrane filter. Cross-flow filtration is highly promising for many winemaking applications but it is quite slow and expensive compared with traditional filtration techniques.