A rheumatologist is a doctor of internal medicine who specializes in arthritis and other musculoskeletal conditions and systemic autoimmune diseases. These diseases can cause pain, swelling, and stiffness in joints, muscles, and bones. Rheumatologists diagnose, treat, and manage a broad range of conditions, including inflammatory (rheumatic) disorders that affect muscles, joints and bones. Connective tissue diseases that affect supporting structures like your ligaments and tendons and may involve the skin and other organs.