A clinical trial is a research study in human patients aimed at answering specific health questions. If a clinical trial is well-conducted, it is the fastest way to find treatments that work in people and ways to improve health. A clinical trial is one of the final stages of a long research process. It determines whether new experimental treatments or new ways of using known therapies are safe and effective, but it must meet ethical and scientific quality requirements. This paper reviews the principal phases of clinical trials and discusses the basic requirements needed to ensure a well-conducted experimental study.