This was the third Horsley lecture given before the Society of Antiquaries of Newcastle upon Tyne in 1947. Building on the speculations and researches of John Horsley, F.G. Simpson and Eric Birley, Stevens develops an intricate argument from the evidence of centurial stones and variations in milecastle and turret design to work out which legions, cohorts and centuries built which parts of the Wall, and to produce a tentative 'labour chart'.