Clinical trials are research studies performed in people that are aimed at evaluating a medical device, drug, or surgical or behavioral intervention. They are the primary way that researchers find out if a new treatment, like a new drug or medical device (e. g., a neuroendovascular device) is safe and effective for patients. A clinical trial is used to confirm if a new treatment is more effective and/or has less harmful side effects than the standard treatment. Other clinical trials test methods of early disease detection, sometimes before there are any symptoms. Still, others test methods to prevent health problems. A clinical trial may also look at how to improve the quality of life in those individuals with a life-threatening disease or chronic health problem. Clinical trials sometimes even study the role of caregivers or support groups. Therefore, clinical trials are very important in the development of medicine. A good clinical trial can influence therapeutic modality and even transform our healthcare work. This book provides a comprehensive review of clinical trials.