Dr. G. Timothy Johnson has been on staff at ABC News for over 25 years, serving as on-air analyst and medical editor. In that role, Dr. Johnson has provided commentary on medical problems and answers for viewers of "Good Morning America," "World News With Charles Gibson," "Nightline" and "20/20."

Dr. Johnson is also founding editor of the Harvard Medical School's "Harvard Health Letter." An accomplished author, he holds a master's degree in public health from Harvard University, and is on the faculty at Harvard Medical School and is a former staff member of Massachusetts General Hospital.