The Framingham Study is a good example of a prospective cohort study. It began in 1948 in Framingham, a Massachusetts town, when all the men aged 45 to 59 were asked to participate, giving a history of their health habits, being examined physically, and giving blood samples. The participants were then reviewed every two years until they died, and disease events were categorized according to the health behaviors elicited in the histories. A similar study for women is the on-going Harvard Nurses’ Health Study begun in 1976 and followed up since then. Much of what is known about how health behaviors affect disease and death have come from these (and similar) studies. Other examples are the three British follow ups of all babies born in a single week in 1948, 1958, and in the late 1960s.