In the United States Navy, commissioned officers are either line officers or staff corps officers. Staff corps officers are specialists in career fields that are professions unto themselves, such as physicians, lawyers, civil engineers, chaplains, and supply specialists. For example, a doctor can advance to become the commanding officer (CO) of a hospital, a hospital ship, or a medical school or the chief of the Bureau of Medicine and Surgery, while a supply officer can become the CO of a supply depot or a school or the head of the Naval Supply Systems Command.