Any architectural intervention in the existing must establish a dialogue with the authenticity of the work. Between constructed material and know-how, this quest for truth is similar for the specialist to defining the fairest gesture, the one that will respect the work of art and history and pass it on to future generations. Faced with the plurality of interpretations of this complex notion of authenticity, but also nourished by the evolution of thoughts and practices, the quest for authenticity may appear to some as a chimera, a rather vague and debatable concept, in particular light of the elements of a temporal stratification retained to project the existing. The opinion of the engineer, the architect and the historian are presented in this book richly illustrated with case studies, and sheds additional light on the definition of a plural and evolving concept