The occurrence of events over time unifies epidemiologic research. Regardless of the study hypothesis and design, the disease-causing actions of exposures and modifying factors are formulated as antecedent to the occurrence of the outcome. All study designs inherently acknowledge time and represent alternative approaches for sampling populations as exposed and nonexposed persons develop disease over time. The cohort design explicitly incorporates the passage of time. In cohort studies, participants are followed over an interval defined by the study's beginning and end, and observations are made on outcome measures of interest: death, incidence of disease, change in a biologic measure, or health status. The study's purpose may be focused—to test a specific (or general) hypothesis—to gather data for descriptive purposes or to facilitate the testing of multiple hypotheses concerning disease. During the follow-up experience of participants in a cohort study, the factors determining the health of the participants may be continually changing as the participants age and exposures to environmental agents start, stop, increase, or decrease (figure 1). The dynamic nature of many risk factors and their relations in time to disease occurrence can only be captured in the cohort design; this temporal interplay is inherently absent from crosssectional data and often investigated only with difficulty using the case-control design (1). Epidemiologists may be challenged in investigating the relation between multiple risk factors, some changing in time, and disease occurrence, as displayed schematically in figure 1. During the follow-up of a cohort, participants age, temporal trends may affect the participants, and exposures to risk factors of primary