This chapter outlines the design and application of cohort studies, which follow groups of individuals over time to examine associations between exposures and outcomes. Cohort studies may be prospective, tracking participants into the future, or retrospective, using historical records. The measure of association most frequently derived from these designs is the relative risk, which quantifies the likelihood of disease in exposed compared to unexposed individuals. Advantages of cohort studies include their ability to establish temporal sequence, study multiple outcomes from a single exposure, and calculate incidence rates. They are particularly useful for investigating common exposures and outcomes with moderate frequency. Limitations include high cost, long follow-up periods, and vulnerability to loss to follow-up and confounding. Practical examples, such as studies linking smoking to lung cancer or occupational exposures to chronic disease, highlight their contribution to public health evidence. Strategies for minimising bias, including careful selection of cohorts, use of standardised data collection, and adjustment for confounders, are discussed. By combining methodological explanation with illustrative examples, the chapter positions cohort studies as powerful tools in epidemiology. This chapter maps to syllabus section 3.2.4, which requires knowledge of cohort designs, measures of risk, and confounding control.