The Grand Ouest is a geographical concept that can varyingly encompass a number of regions located in the western third or half of France, in the Atlantic Arc.
This territorial grouping is not formally defined and its many definitions are due in particular to the fact that it has never had its own political and administrative structure.
Not referring to any official subdivision or territorial authority, the Grand Ouest is an entity with variable definitions. It systematically includes Brittany and the Pays de la Loire, frequently the former Lower Normandy and Poitou-Charentes regions, and sometimes extends to the Centre-Val de Loire region and the former Upper Normandy region.
The territory is often the subject of projects and facilities shared by the different regions concerned (such as the Cancéropôle du Grand Ouest); sociological, political and economic areas of study ; or technical and administrative areas (such as the tissue bank of the ). However, in some cases, the term âÂÂOuestâ is used to define this geographical area (for example, for the former European electoral constituency and INSEE's ZEAT Ouest). Its two largest cities, by any definition, are Nantes and Rennes.