The St William Window at York Minster is one of the greatest surviving English works of art of the late medieval era. It forms one of three monumental stained-glass narratives created at York Minster in the first decades of the fifteenth century, and like its world-famous neighbour the Great East Window, it is the work of the master glazier John Thornton. The window's monumental scale is matched by its breathtaking artistry, which bears comparison with the finest surviving artistic representations of saints' life cycles of the age.The window's subject is the life and miracles of St William of York, a not uncontroversial twelfth-century archbishop whose shrine within the Minster became the focus of a major medieval miracle cult. The modern conservation of the window, which was underpinned by ground-breaking historical research, has enabled the unravelling of its complex narrative and allowed the conserved glass to be reassembled correctly for the first time since the window's creation. This remarkable conservation story has never previously been told.