Since the publication of the Venice Charter, authenticity has been a central guiding principle of heritage conservation. Originally referring almost exclusively to tangible substance, the concept has recently been extended to include intangible values as an expression of cultural significance. However, the term is still far from being well defined: Authenticity is a fragile construction that is constantly being renegotiated. Focusing on the High Modern period, this volume examines in particular the authenticity of a monument's technical nature. What are the specific communicators of cultural significance to be preserved in such a case, and what does this mean for the treatment of a building's structure, envelope and interior installations? And shouldn't even the typical defects of the period or an ingenious structural system be considered as expressions of their time's art of engineering, worthy of protection?