In this course the principles and practice of cohort, case-control and cross-sectional studies are taught. Design, data collection and outcome measures are discussed, as well as the major advantages and disadvantages of the different study designs. Emphasis is on the application of study design in etiologic research. The course focuses on the more classical approach but also addresses modern concepts and design options, such as case-cohort designs. Various study designs are discussed, with emphasis on issues of validity.