Drainage is the runoff of water from an area, either naturally or, in agriculture, under artificial control. In nature, drainage generally takes the form of a pattern of streams, which feed rivers and lakes and flow, usually, to the sea (see water cycle). An area all of whose rainwater drains into a particular body of water is called a watershed or catchment basin (see groundwater).

Systems of artificial drainage depend on on the nature of the soil, as well as local topography. Two main systems are used: surface and subsurface. Surface systems usually comprise a pattern of ditches; subsurface systems a pattern of conduits and tunnels. These usually lead to a natural stream. See also irrigation.