Arguments about the origins and purpose of the dykes have raged since antiquarians first described them but they have been neglected in recent studies of the Dark Ages. Our understanding of the landscape in which the dykes were built has radically altered over the last fifty years due to advances in archaeological techniques and methods yet much of this information has not previously been applied to studies of the dykes. This book discusses how the dykes could have functioned and possible reasons why they were built as well as the development of ideas about the dykes. A survey and catalogue of all the known post-Roman dykes is also included.