This chapter discusses the clinical trial as “an experiment that studies the relationship between a disease or condition and a medical intervention for prevention, diagnosis, or treatment using human subjects.” The ultimate goal of a clinical trial should be to advance medical knowledge and to improve medical practice to benefit the patients. Hill conducted two types of clinical studies, both involving human subjects; however, the first study, linking smoking and lung cancer, was based on observational investigations, and the second, prospectively testing streptomycin in the treatment of pulmonary tuberculosis, was an experiment, that is, a clinical trial. Clinical studies conducted in medical schools include retrospective surveys of clinical data or accumulated case reports, which are often cross-sectional, as well as clinical trials, which are always prospective and thus require both a baseline and a follow-up period. The protocol for the trial is the central document that guides the study throughout the course of the work and serves as the roadmap.