The National Institute of Health defines clinical trials as biomedical or health-related research studies in human beings that follow a pre-defined protocol. Clinical trials involve human beings that have a disease, but may also involve healthy volunteers. An investigational item is usually being studied. Clinical Trials can be Phase 1, 2, 3 or 4 depending on the stage of experimentation on the investigational item (i.e. device or drug).

Each study answers scientific questions and tries to find better ways to prevent, screen for, diagnose or treat a disease. Clinical trials may also compare a new treatment to a treatment that is already available. Every clinical trial has a protocol, or action plan, for conducting the trial. The plan describes what will be done in the study, how it will be conducted, and why each part of the study is necessary. Each study has its own rules about who can participate. Some studies need volunteers with a certain disease. Some need healthy people. Others want just men or just women.