Clinical trials are research studies designed to assess the efficacy and safety of an intervention. The intervention is most often a drug but can also be a device, such as a pacemaker or stent, a surgical procedure, or a diagnostic tool, such as a blood test. Clinical trials typically involve patient volunteers, but sometimes they involve healthy subject volunteers. Thousands of clinical trials are conducted each year around the world and may take place at a variety of locations, including universities, hospitals, clinics, physicians’ private offices, and professional clinical research sites. In the beginning of 2021, there were over 374,000 studies registered with ClinicalTrials.gov, a U.S. National Library of Medicine database of privately and publicly funded clinical studies conducted around the world. Several thousand patients or more typically participate in clinical trials for each new drug before it becomes available to the general public.