This chapter provides an overview of epidemiological study designs, outlining their classification and the principles that guide their selection and application. It begins with descriptive designs, including case reports, case series, and cross-sectional studies, which are used to generate hypotheses and describe patterns of disease. It then examines ecologic studies and a range of analytical designs, such as case-control, nested case-control, case-cohort, and case-crossover studies, which assess associations between exposures and outcomes. The discussion extends to cohort studies—both prospective and retrospective—that allow for temporal assessment of risk. Experimental designs are also covered, focusing on randomized controlled trials (RCTs), including crossover and factorial trials, as well as the sequential phases of clinical trials. By comparing strengths, limitations, and appropriate applications, this chapter equips readers with the knowledge to critically select and interpret study designs in epidemiological research.