This article examines the use of glass historically in the preservation of monuments and in building in the existing fabric (Bauen im Bestand). For obvious reasons, glass was used early on as part of the protection of sculptures, archaeological sites and architectural monuments: it offers protection from the weather, and is translucent and transparent, thereby providing the necessary protection without obstructing the view of the conserved work. Between the often alleged invisibility of interventions in glass in historical contexts (‘it’s only glass’) on the one hand and the desire to highlight the contrast between old and new on the other, there appear to be rather contradictory explanations for the ideas behind the use of glass in historical contexts. Indeed, architects as well as preservationists soon appreciated the possibility of making additions to monuments out of glass, precisely because these could thus be distinguished as contributions true to their own era, while at the same time conveying respect for existing fabric. This attitude, which has fallen somewhat out of fashion in recent years, reached its peak between the 1960s and 1980s, when the contrast of new glass in front of old masonry became a leitmotif of architecture in the historic environment. In this context, transparency was no longer the only mode of expression: mirror effects also began to enjoy great popularity from the 1970s, and the soon ubiquitous ‘glass joint’ marked the approved dividing line between old and new. Particularly vivid examples also demonstrate how such an updating of historic architecture, with its simultaneous emphasis on a monument’s past and present, could also be useful in a political context-regardless of whether the accent lay more on the continuity of the institution or on a questioning of the monument’s history.