The gigantic barns built by the major landowners of medieval England are among our most important historic monuments. Impressive structurally and architecturally, they have much to tell us about the technology of the time and its development, and are buildings of great and simple beauty. But, unlike houses, castles and churches, barns were centres of production, where grain crops were stored and threshed, and allow us to glimpse a very different side of medieval life - the ceaseless round of the farming year on which the lives of rich and poor depended. The Great Barn at Harmondsworth, built in 1425-7 for Winchester College, rescued and restored by English Heritage and Historic England in the last decade, is one of the most impressive and interesting of them all. Prefaced by an exploration of the ancient estate to which it belonged and of its precursor buildings, this book explores why, how and when the barn was built, the ingenuity and oddities of its construction, and the trades, materials and people involved. Aided by an exceptionally full series of medieval accounts, it then examines the way the barn was actually used, and the equipment, personnel, processes and accounting procedures involved - specifically relating to Harmondsworth, but largely common to all great barns. Finally, it covers its later history, uses and ownership, and the development of scholarly and antiquarian interest in this remarkable building.