A clinical study involves research using human volunteers (also called participants) that is intended to add to medical knowledge. There are two main types of clinical studies: (1) clinical trials (also called interventional studies) and (2) observational studies.

Clinical trials apply the scientific method to human health. Observational studies are those in which individuals are observed and their outcomes are measured by the investigators. Interventional studies are those in which the research subjects are assigned by the investigator to a treatment or other intervention, and their outcomes are measured.

A principal investigator, who is often a medical doctor, leads every clinical study. Clinical studies also have a research team that may include doctors, nurses, social workers, and other health care professionals.