Many natural and artificial materials exhibit selective permeability, meaning that only molecules or ions of a certain size, shape, polarity, charge, etc., can pass through (diffuse) the material. Biological cell membranes provide elegant examples of selective permeation in nature, but a simpler technological example is the dialysis tubing used to remove metabolic wastes from blood. Regardless of how they are made, these materials are commonly referred to as semipermeable membranes.

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In the process of osmosis, diffusion serves to move water through a less to semipermeable membrane a concentrated solution is a more concentrated solution. Osmotic pressure is the total pressure a more concentrated solution must be applied to stop osmosis. As more pressure is applied, water moves from a more concentrated solution to a less concentrated (more pure) solution. This is called reverse osmosis. Reverse osmosis (RO) is used to purify water in many applications, from desalination plants in coastal cities, to water purifiers in grocery stores, and small reverse osmosis household units. With a hand-operated pump, small RO units can be used in third world countries, disaster areas and lifeboats. Our armed forces have a variety of generator-operated RO units that can be transported to remote locations in vehicles.