The Knights of St John, or the Knights Hospitaller, were one of the most famous Christian military orders. Their humble origins lay in helping early pilgrims at Jerusalem from the turn of the 12th century, but they developed into a true multi-national organisation with headquarters in almost all European countries. The Priory of England was centred at Clerkenwell, London, where the surviving medieval crypt and Tudor gatehouse are well-known landmarks. Several large-scale excavations by the Museum of London in the 1980s and 90s have been combined with antiquarian surveys in this monograph to produce a remarkable picture of a priory. Founded in 1144, this highly unusual religious house evolved from a round-naved church and associated buildings into one of London's premier palatial residences. Unusually, the priory's buildings were retained after the Dissolution of the Hospitallers in 1540, becoming the residence of nobility and the location, for a while, of the Royal Master of the Revels. There was only ever one headquarters of the Knights Hospitaller in England: this book provides a fascinating blend of the history and archaeology of a unique site.