This volume examines the 20th-century defence of Britain as part of a series covering military archaeology in the United Kingdom. The project was a response to growing concern about Britain's recent defence heritage and about the rate at which many historically significant structures were succumbing to demolition or neglect. The project attempts to verify and augment archival research using fieldwork conducted by members of the public from all walks of life. Its objective is to create a record of Britain's military defensive efforts in the present century, which should inform conservation strategies and encourage future research.