A pediatrician is a doctor who treats and works with children. They provide preventative medicine and medical care for children who are ill or dealing with a chronic condition. They are concerned with the physical, emotional, and mental well-being of the children they treat. They will work with their patients as they progress through each stage of development and address any issues which may arise during that time. Pediatricians focus on infants and children through age 21 and will help them to foster healthy lifestyle habits.