The flow of two fluids in contact with each other or separated by a surface but flowing in opposite directions. In a distillation or adsorption column, the flow of liquid descends while the flow of vapour ascends, meeting in intimate contact. In a heat exchanger, the two fluids travelling in opposite directions are not in direct contact with each other, but exchange heat from one to the other through the tube walls. This arrangement is more efficient than cocurrent flow, in which the flows are in same direction.