From the Wikipedia page 
Essonne (pronounced [ɛsɔn]) is a French department in the region of Île-de-France. It is named after the Essonne River.
It was formed on 1 January 1968 when Seine-et-Oise was split into smaller departments.
Essonne belongs to the region of Île-de-France.
It has borders with the departments of:
- Hauts-de-Seine and Val-de-Marne to the north,
- Seine-et-Marne to the east,
- Loiret to the south,
- Eure-et-Loir and Yvelines to the west.
All of northern Essonne department belongs to the Parisian agglomeration and is very urbanized. The south remains rural.