Cohort studies are the second main type of observational study design in epidemiology.  This design is desirable when there are many potential outcomes you want to investigate, when the goal is a direct measure of incidence or risk, and for rare exposures.  Cohort studies can be prospective, where the investigators assemble the cohort and then follow them to observe outcomes, or retrospective when the investigators identify the cohort based on past exposures and evaluate outcomes that have already occurred. Prospective cohort studies are less vulnerable to bias and can evaluate the temporal relationship between exposure and outcome.  Retrospective studies are good for diseases with long induction and latent periods.  Some weaknesses of cohort studies are that they are inefficient for rare outcomes, can be very expensive and time-consuming, and retrospective cohort studies can be more prone to bias. There are many examples of large and small cohort studies, some of which have been going on for decades, and there are still many questions that can be answered from these studies!