This chapter illustrates the study designs most frequently encountered in the medical literature. In medical research, either subjects are observed or experiments are undertaken. Experiments involving humans are called trials. Experimental studies may also use animals and tissue, although we did not discuss them as a separate category; the comments pertaining to clinical trials are relevant to animal and tissue studies as well. Each type of study discussed has advantages and disadvantages. Randomized, controlled clinical trials are the most powerful designs possible in medical research, but they are often expensive and time-consuming. Well-designed observational studies can provide useful insights on disease causation, even though they do not constitute proof of causes. Cohort studies are best for studying the natural progression of disease or risk factors for disease; case-control studies are much quicker and less expensive. Cross-sectional studies provide a snapshot of a disease or condition at one time, and we must be cautious in inferring disease progression from them. Surveys, if properly done. are useful in obtaining current opinions and practices. Case-series studies should be used only to raise questions for further research.