Rheumatology is a field of medicine that is concerned with the evaluation and treatment of people with autoimmune conditions and joint diseases, including arthritis. A rheumatologist is a physician who specializes in the field of rheumatology. Rheumatologists are internists who are qualified by additional postgraduate training and experience in the diagnosis and treatment of arthritis and other diseases of the joints, muscles and bones. Rheumatologists treat arthritis, autoimmune diseases, pain disorders affecting joints, and osteoporosis. There are more than 200 types of these diseases, including rheumatoid arthritis, osteoarthritis, gout, lupus, back pain, osteoporosis, and tendinitis. Some of these are very serious diseases that can be difficult to diagnose and treat.