Cohort studies provide the most comprehensive approach for evaluating overall patterns of health and disease. Every epidemiologic study should be based on a particular population's experience over time. The cohort design has the advantage, relative to other study designs, that ideally it includes all of the relevant person-time experience of the population under study, whereas other study designs, such as case–control and cross-sectional studies, involve sampling from that experience. However, cohort studies, particularly prospective studies, may be relatively expensive in terms of time and resources, and typically involve the efforts of epidemiologists, environmental scientists, occupational hygienists, biostatisticians, computer programmers, and clerical staff. The specific features of cohort studies that we present in this chapter include cohort enumeration, follow-up procedures, the choice of comparison populations, person-time analysis, stratified analysis, subcohort analyses, analyses of cumulative exposure, and latency and induction time.