Guided by the hypothesis that the doctrine of Historic Monuments - between theoretical positions, subjective assessments and opportunistic arbitrations - has been the blind spot in restoration in France since the end of the 19th century, the objective of this doctoral thesis is to reopen this issue, based on the texts and decisions of Paul Léon (1874-1962), the main head of the historic monuments Department in the interwar period. Although his role is, today, often forgotten. In fact, Paul Léon has had an exemplary career at the Fine-Arts. He was head of the Historic Monuments Department from 1907 to 1932, a member of the Commission from 1907 to 1962 and Director of Fine Arts from 1919 to 1932. He is also the historiographer of the Departme...