Les Ulis is a commune in the Essonne department located in the southwestern suburbs (banlieue) of Paris, France. It is 23km from the centre of Paris.Inhabitants of Les Ulis are known as Ulissiens.LocationLes Ulis is a new town located in the Île-de-France, in the south-west of the Parisian agglomeration and in the north-west of the Essonne department. It sits on the Cortabœuf plateau which dominates the Yvette valley, also known as the Chevreuse valley. It is 23km to the south-west of central Paris, 20 km north-west of Évry and 14 km to the south-east of Versailles.GeographyThe town is laid out in the form of a U, approximately three km by two km, and covers 680 hectares. Over 65% of the area is built-up, 17% is semi-rural and 16% is either rural or comprises communal green space. The French Institut Géographique National gives the co-ordinates of the town as 48°40'58" N and 02°09'55" E at the central point.The source of the minor river Rouillon (river) is in the neighbouring town of Villejust, close to the communal border. The lowest point of the town, at the point where it meets Bures-sur-Yvette, is at an altitude of 87m. Most of the town is situated at an average altitude of over 150m. Its highest point is at 170m. The land on which it is built consists of clay, sand and millstone.