This text deals with the design and accuracy of epidemiological studies. It aims to provide students with a clear understanding of the research strategies used in experimental studies so that they may ask the right questions, select the study population and follow-up period, obtain information on exposures, and follow up disease occurrences. Each step in the planning or critical evaluation of a study is illustrated by exercises based on actual cases. These exercises cover a wide range of exposures (including dietary factors, tobacco, alcohol, drugs and radiation and occupational hazards) and diseases.