Nutrient retention mechanisms in riparian buffer strips are reviewed with emphasis on surface runoff and subsurface flows, the main pathways of exchanges between the stream and its surroundings. Unique physical and biogeochemical properties prevailing in these riparian ecotones dictate the flux of water, nutrients and other exogenous substances between the upland areas and the stream. Removal of nutrients from surface inflows is induced by deposition of sediment bound nutrients and exchange of dissolved nutrients with the soil/litter surface. Removal of nitrogen in subsurface flows can partly be explained by vegetation uptake, but the main mechanism for removal is usually denitrification. In channelized streams, the subsurface inflows have, in most cases, been altered to discharges via drainage tiles, with the exchange flows (water leaving and re-entering the open-channel via the stream bed and banks) being greatly decreased. Consequently, to improve nitrogen removal in these systems, these flows have to be intercepted or reestablished either through restoration of the old stream valley or through managed structures in the buffer strips.