The pillboxes of Britain and Ireland are among the most important military structures employed in the history of the defence of these islands. This work presents the first thorough study and classification of pillboxes and related structures, including selection posts, Seagull and concrete trenches, gun-houses and turrets, battle headquarters and spigot-mortar emplacements. The author traces the use of small, free-standing defence structures from ancient times to the present, placing the pillbox within a historical continuum and identifying its course of development. The work also presents a typology of British and Irish pillboxes through a catalogue of known types and individual examples, and examines their tactical employment in the landscape.