Hemophilia is a set of potentially life-threatening genetic blood clotting disorders. It is relatively rare, afflicting about 1 out of 7,000 people. Yet, among Queen Victoria and her children, grandchildren, and great-grandchildren, there were 15 individuals who either carried or displayed the disease. We will first look at the biology of hemophilia and how it is genetically transmitted.  Then we will explore how this disease affected the royal families of England, Russia, Spain, and Prussia and how it influenced the course of European history.