Clinical trial: A study that is intended to evaluate the safety and effectiveness of medications or medical devices by monitoring their effects on large groups of people. Studies may be conducted by government health agencies (such as the National Institutes of Health [NIH]), researchers affiliated with hospital or university medical programs, independent researchers, or individuals from private industry. Usually volunteers are recruited, although in some cases research participants may be paid. For some patients, clinical research trials represent an avenue for receiving promising new therapies that would not otherwise be available. Patients with difficult-to-treat or 'incurable' diseases may pursue participation in clinical research trials if standard therapies are not effective.