During the nineteenth century, children's physical health became a dominant theme in France and Great Britain, two of Europe's pediatric pioneers. This dissertation examines how British and French doctors, legislators, hospital administrators, and social reformers came to see the preservation of children's physical health as an object of national and international concern. Medical knowledge and practice shaped, and was shaped by, nineteenth-century child preservation activities in France and Great Britain, linking medicine, public health, and national public and private efforts to improve the health of nations, especially that of their future members. Children's hospitals played a significant role in this process by promoting child health; preventing and combating childhood diseases; fostering pediatric professionalization and specialization; and diffusing medical-based justifications for child welfare reforms in the second half of the century. This deeply contextualized tale of two hospitals, Great Ormond Street Hospital for Children in London (1852) and Sainte-Eugénie in Paris (1855), traces a crescendo in the interest, provision, and advocacy for children's medical care over time: from foundling homes and dispensaries to specialized hospitals with convalescent branches and large outpatient clinics. As a comparative study of the medicalization of children's bodies between 1820 and 1890, this dissertation also investigates the transnational exchange of medical ideas, institutions, and practices pertaining to child health between France and Great Britain during a period of nation-building. Specialized pediatric institutions in Paris and London built upon and solidified local, national, and international interests in improving and preserving child health. Despite great differences in their hospital systems, French and British children's hospital administrators and doctors looked to one another as partners, models, and competitors. Nineteenth-century French and British concerns for national public health, and child health in particular, had important distinctions and parallels, but medical, institutional, and legislative developments related to these concerns were not isolated activities, but rather, tied to transnational communication, cooperation, and competition.