This is the first attempt to investigate the background to the gift of the Holy Blood to Westminster by King Henry III in 1247. Recently the archives of Westminster Abbey have yielded important new material relating to this extraordinary event, including a letter from the patriarch of Jerusalem that describes the relic in detail. This study offers both a commentary on this newly-discovered letter, and an overview of the extraordinary history of the relics of Christ's blood, their origin, distribution, and place in popular devotion.