Ille-et-Vilaine (French pronunciation: [il e vilɛn] ⓘ; Gallo: Ill-e-Vilaenn, Breton: Il-ha-Gwilen) is a department of France, located in the region of Brittany in the northwest of the country.

Other important rivers include: The department is moderately elevated above the level of the sea, with many hills; however the central part has a dense network of many tributaries to the Ille or the Vilaine from all around the large basin of Rennes.

The elevated hills bordering this basin are covered by several old forests now exploited by men for the production of wood.

To avoid these hazards within inhabited cities, some natural fields bordering the Vilaine in the south of the department are now left floodable, and works for regulating the level have been done including, small artificial lakes with derivation channels, replanting trees in the basin, better management of forests, and regulating the artificial drains made for agriculture.

Since then, the growth of middle-class suburbs have helped the Socialists, who have been rapidly gaining strength in those formerly right-leaning areas.