Population studies in which the subjects (also referred to as the cohort) are followed for some time (follow-up period). At the start of the study (also called the baseline), the cohort consists of people who are free of the disease under investigation and differ in exposure conditions. All persons are examined and information on variables of interest is collected. During the course of the study, the occurrence of disease is recorded. From this, the incidence of the disease in the study population can be calculated. Based on these data, inferences on the association between exposure and occurrence of disease can be drawn. An important advantage of a follow-up study is that exposure is measured before the disease has developed. The appropriate follow-up period depends on the associations that are studied. Because the majority of follow-up studies concern chronic diseases, the follow-up period is usually long. Consequently, results are only available after many years. Furthermore, for the assessment of associations between exposure and disease, it is necessary for the number of cases that manifest themselves during the follow-up period to be sufficiently large. This means that the cohort approach is not suitable for studying rare diseases.