The term "cohort study" was introduced by Frost in 1935 to describe a study that compared the disease experience of people born at different periods, in particular the sex and age specific incidence of tuberculosis and the method was extended to the study of non-communicable disease by Korteweg who used it 20 years later to analyse the epidemic of lung cancer in the Netherlands. Such studies are now best described as generation studies or generation cohort studies to distinguish them from the common type of study that is now carried out that consists in defining groups of individuals distinguished by some variable (such as place of residence, occupation, behaviour, or environmental exposure) and following them up to see if the incidence or mortality rates vary with the selected variable. This type of study is now one of the most important tools for epidemiological investigation. Initially called prospective studies, because the information characterising the individuals in the cohorts was recorded before the onset of disease, they are now preferably called cohort studies and distinguished as prospective cohort studies, If the information obtained relates to the subjects at the time the study is started and they are then followed, or retrospective cohort studies, if the information characterising the individuals was recorded sometime in the past (for example, the receipt of radiotherapy, or entry to a specific occupation). Studies of either type have the great advantage that they avoid all the most important sources of bias that may affect case-control-studies, but the disadvantage that because incidence rates and more specifically mortality rates are commonly low, large numbers of subjects have to be followed for several (if not many) years to obtain statistically significant results. Several early prospective studies are described: Namely, those of 34,000 male British doctors, 190,000 male and female American citizens with different smoking habits, some 5,000 middle aged residents of Framingham with different blood pressures, blood cholesterol levels, etc, and 13,000 children born in the UK in one week in 1946 with different family backgrounds.