Proponents of evidence-based medicine would acknowledge that several sources of evidence inform clinical decision making. Hierarchies of evidence have, however, been developed to help describe the quality of evidence that may be found to answer our clinical questions. According to this classification, the randomized clinical trial is the most effective way to determine whether a cause-and-effect relationship exists between an intervention and a predefined outcome. The basic principles of clinical trial design are reviewed, and the unique challenges of trial design in surgery are discussed.