The story of blood is a story of transformation, divided into three eras. The first period, from antiquity to the early twentieth century, involves the transformation of blood from a magical substance - the blood of Christ as holy sacrament - to a component of human anatomy, capable of being studied and turned into a source of healing. In the next era, which lasts until the end of the Second World War, the scientific curiosity of blood becomes a strategic material. We see medical scientists master the resource, learning the techniques of mass collections and storage, ironically aided in their research by the battlefield necessities of two world wars. In the final period, the global traffic in blood-products explodes, saving the lives of untold transfusion recipients and haemophiliacs, but resulting in some of the worst medical calamities western science has known.Is blood a gift of charity or a pharmaceutical product, no more precious than, say, penicillin? The way in which societies decide to answer this question will determine how we deal with this precious yet dangerous resource in the years to come.