A louver (American English) or louvre (British English), is a window blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain, direct sunshine, and noise. The angle of the slats may be adjustable, usually in blinds and windows, or fixed.
Louvers originated in the Middle Ages as lantern-like constructions in wood that were fitted on top of roof holes in large kitchens to allow ventilation while keeping out rain and snow. They were originally rather crude constructions consisting merely of a barrel. Later they evolved into more elaborate designs made of pottery, taking the shape of faces where the smoke and steam from cooking would pour out through the eyes and mouth, or into constructions that were more like modern louvers, with slats that could be opened or closed by pulling on a string.
Modern louvers are often made of aluminum, metal, wood, or glass. They may be opened and closed with a metal lever, pulleys, or through motorized operators.
Some modern louver systems serve to improve indoor daylighting. Fixed mirrored louver systems can limit glare and of redirect diffuse light. Such louvers may be integrated in between two panes of double glazing.
Louvers are rarely seen as primary design elements in the language of modern architecture, but rather simply a technical device. However, there are examples of architects who use them as part of the overall aesthetic effect of their buildings. The most well-known example is Finnish modernist architect Alvar Aalto who would create aesthetic effects in the facades of his buildings through the combination of different types and sizes of louvers, some fixed some moveable, and made mostly from wood (e.g., the various buildings of the Helsinki University of Technology). A second example, taking influence from Aalto, is the second-generation modernist architect Juha Leiviskä.
In industrial facilities such as Steel Foundries and Power Plants, louvers are very common. They are utilized for natural ventilation and temperature control.
Flood louvers or flood vents are openings at the base of a building, covered by one or more moving flaps. They are designed to allow floodwaters to enter and leave the building, equalizing hydrostatic pressure on the walls and mitigating structural damage due to flooding.
Louvers are used as semi-passive means of thermal control on spacecraft as well. They are also available as an accessory for some automobiles.
See also 
- Henisch (1976), pp. 96–97.
- Dariusz Heim and Kamil Kieszkowski: Shading Devices Designed to Achieve the Desired Quality of Internal Daylight Environment, PLEA2006 - The 23rd Conference on Passive and Low Energy Architecture, Geneva, Switzerland, 6-8 September 2006