Îles Saint-Marcouf comprise two small uninhabited islands off the coast of Normandy, France. They lie in the Baie de la Seine region of the English Channel and are 6.5 km (4.0 mi) east of the coast of the Cotentin peninsula at Ravenoville and 13 km (8 mi) from the island of Tatihou and the harbour at Saint-Vaast-la-Hougue. In addition to the fortifications described below, on the larger island there is a lighthouse that dates to 1948.

The Cotentin Peninsula, also known as the Cherbourg Peninsula, is a peninsula in Normandy that forms part of the northwest coast of France. It extends north-westward into the English Channel, towards Great Britain. To its west lie the Gulf of Saint-Malo and the Channel Islands, and to the southwest lies the peninsula of Brittany.