Countercurrent exchange is a mechanism used to transfer some component of a fluid from one flowing current of fluid to another across a permeable barrier between them. It is used extensively in biological systems for a wide variety of purposes (it is also a key concept in chemical engineering thermodynamics). For example, fish use countercurrent exchange in their gills to transfer oxygen from the surrounding water into their blood, and birds use a countercurrent heat exchanger between blood vessels in their legs to keep heat concentrated within their bodies.

The diagram to the right presents a generic representation of a countercurrent exchange system, with two parallel tubes containing fluid separated by a permeable barrier. The substance to be exchanged, whose concentration is represented by the darkness of the orange shading, transfers across the barrier in the direction from greater concentration to lesser concentration. With the two flows moving in opposite directions, the countercurrent exchange system is able to maintain a constant concentration gradient between the two flows over their entire length, and can result in almost all of the substance being transferred.

Contrasting with the countercurrent exchange system is the concurrent exchange system, in which the two fluid flows are in the same direction. As the diagram to the left shows, a concurrent exchange system is only capable of moving half of the substance across from one flow to the other, no matter how long the exchanger is.