A project without memory is an amputated project: history must be considered both as a methodological basis and as a veritable tool for work. Underlying all intervention existent, extensive historical and material knowledge of what is built must guide the choices of the project makers for a creative approach, which, combining theoretical knowledge and technical knowhow, demonstrates the disciplinary coherence of the selected options. These intentions, synthesized in the idea of the material history of modern and contemporary building constitute a precious key to the reading of projects for existing national heritage, as much in terms of method as in their cultural implications. The engagement of the laboratory of Techniques et de la Sauvegarde de l'Architecture Moderne of the Ecole Polytechnique Federale in Lausanne (EPFL-TSAM) in the mission as observatory of practices of preservation of national heritage of the 20th century has, without doctrinarian leanings, led to discern major tendencies in the practice, situating them in the rich debate engaged since the end of the 18th century. With the notion of authenticity as a background, a veritable material history of the project of preservation deserves to be written today.