In March 1946, scientists began to track thousands of children born in one cold week. No one imagined that this would become the longest-running study of human development in the world, growing to encompass five generations of children. Today they are some of the best-studied people on the planet, and the simple act of observing human life has changed the way we are born, schooled, parent, and die. This is the tale of these studies and the remarkable discoveries that have come from them. Touching almost every person in Britain today, they are one of our best-kept secrets.