It is by far the largest island in the Thames Estuary and the fourth largest in England, measuring 5 miles by 3 miles.
Foulness lies on the edge of the huge Maplin Sands, and has been through most of the 20th century used for military purposes by the Ministry of Defence.
There are two tiny villages, Churchend, where the parish church was built in 1850, and Courtsend. Until the 20th century, the only access to the island was at low tide by means of the Broomway, a causeway across the sands marked by brooms. There is now a road from Great Wakering across a series of bridges.
Foulness is well known as a breeding ground for birds, including the ten thousand Brent geese who visit annually.
In the 1970s a proposal to build a new London Airport, plus docks, on Foulness and the Maplin Sands was eventually abandoned.
Apart from the small population of around two hundred, access to the island is generally not allowed.