Considered a monument of early Modernism and one of the pioneering examples of modern academic architecture, the Bauhaus Dessau is a World Heritage Site that experienced three main phases of renovations: in 1976, between 1996-2006 and 2009-2012. This thesis examines the alterations of the building’s windows as part of these renovations in order to explore challenges related to sustainable retrofits in modern architecture. Specific emphasis is given on windows based on their critical role in energy performance as well as their extensive use throughout the building, making them character-defining elements for the Bauhaus. Preservation strategies are identified and analyzed from the perspectives of historic preservation and sustainability, following an overview of Modernism’s relationship to sustainable preservation and an introduction of the history of the site as well as its climate conditions. Changes of window elements in three major building parts, the Studio Wing, the Workshop Wing and the North Wing, are discussed to demonstrate the challenges of achieving improved energy efficiency with the least possible compromise from character-defining elements. Through the analysis of historic documentation, existing literature and energy management plans in addition to interviews with multiple stakeholders, this study explores the preservation decisions that were critical in the renovations of the windows. The results of the study illustrate that, although challenges common in preserving modern architecture were creatively overcome at the Bauhaus Dessau, there is still a need to consider future climate change scenarios.