A selectively permeable membrane is a thin barrier which allows passage of some substances through it, but prohibits the transport of others, based on specific criteria, such as size, charge, affinity (or lack thereof) for certain substances, structure, or presence of chemical bonds. Osmosis is an example of passive transport over a selectively permeable membrane, and filtration is an example of active transport over a selectively permeable membrane.

Selectively permeable membranes are found throughout the field of biology: the cell's plasma membrane is one, one's intestines are another, there are such membranes in the lungs which allow exchange of oxygen and carbon dioxide with the blood. Mechanical filters usually operate on this principle. Think of a selectively permeable membrane as a wall with holes in it. Small stuff may be able to go right through it, while big stuff has no chance. There might be other factors affecting what stuff goes through: a square peg might not go through a membrane with round holes.

See osmosis for an interesting demonstration of selectively permeable membranes.