Complementing the chapter on clinical trails, this chapter provides an overview of how to design observational analytic epidemiological studies for infectious disease prevention and. The appropriate study is a balance between rigour, appropriateness, and feasibility. Key threads throughout the chapter include how to construct the aims and objectives, the main types of design, their rigour compared to trials, and how to choose the correct design to fulfil the objectives. The main features and results to be obtained from cohort, case control, cross-sectional, ecological, and descriptive studies are described. Variations in these designs are also touched on. A final section covers how to judge the evidence from a study for its validity and generalizability and, if relevant, how to use such evidence with other information to determine a causal link between a risk factor and disease.