A rain gutter, eavestrough, eaves-shoot or surface water collection channel is a component of a water discharge system for a building. It is necessary to prevent water dripping or flowing off roofs in an uncontrolled manner for several reasons: to prevent it damaging the walls, drenching persons standing below or entering the building, and to direct the water to a suitable disposal site where it will not damage the foundations of the building. In the case of a flat roof, removal of water is essential to prevent water ingress and to prevent a build-up of excessive weight.

Water from a pitched roof flows down into a valley gutter, a parapet gutter or an eaves gutter. An eaves gutter is also known as an eavestrough (especially in Canada), spouting in New Zealand, rhone (Scotland), eaves-shoot (Ireland) eaves channel, dripster, guttering, rainspouting or simply as a gutter. The word gutter derives from Latin gutta (noun), meaning "a droplet".

Guttering in its earliest form consisted of lined wooden or stone troughs. Lead was a popular liner and is still used in pitched valley gutters. Many materials have been used to make guttering: cast iron, asbestos cement, UPVC (PVCu), cast and extruded aluminium, galvanized steel, wood, copper, zinc, and bamboo.