It lies some 27 kilometres (17 mi) northeast of Chartres, at the confluence of the Drouette and the Guesle.

In the 11th century, they raised the fortresses of Épernon and Montfort for the protection of the Château de St Léger and granted a charter to the town.

In the 13th century it became an independent lordship, which remained attached to the crown of Navarre till, in the 16th century, it was sold by Henry III of France to Jean Louis de Nogaret de La Valette, for whom it was raised to the rank of a duchy in 1581.

De Nogaret's second son, Bernard (1592–1661), succeeded his father to the dukedom and after his death, the title was borne by the families of Goth and of Pardaillan.

[4] The town is endowed with one secondary school named after the French mathematician Michel Chasles and which was founded in 1973.