Mayo Clinic is a not-for-profit medical practice and medical research group specializing in treating difficult patients (tertiary care). Patients are referred to Mayo Clinic from across the U.S. and the world, and it is known for innovative and effective treatments. Mayo Clinic is known for being at the top of most accredited quality standard listings. For example, it has been near the top of the US News and World Report List of Best Hospitals for more than 20 years.