This thesis aims to investigate how the meaning of the concept of authenticity has been used in arguments about the reconstruction of Nationalmuseum in Stockholm, Sweden. The museum, which opened for the first time in 1866, was closed for restoration, renovation and other engagements during 2013-2018. After the reopening, a debate took place in the Swedish newspaper Svenska Dagbladet. In the debate there were people who argued that the reconstruction had erased the museum’s history and replaced it with modern design. Other people argued that the reconstruction had made the building more true to its history as older reconstructions had distorted the original building. The main source material for the present study consists of a final report of the project made by the National Property Board, selected debate articles from the debate on the reconstruction in Svenska Dagbladet, and an interview and two mail conversations that have been made with persons involved in the project. Through argumentation analysis, arguments from the final report, the debate in Svenska Dagbladet, the interview and the mail conversations have been analyzed. The arguments selected from the source material are arguments that either indicate that the authenticity has been preserved or that the authenticity has not been preserved. The thesis does not intend to determine whether the reconstruction of Nationalmuseum has preserved the authenticity or not, it is the argumentation about this issue that is to be investigated. The reason is that the perception differs between different people, different contexts and different views. The analysis shows that the arguments claiming that the authenticity has not been preserved above all relate to the building's younger reconstructions with its additions that were removed during the current reconstruction. Some debaters argue that the building's younger history also has a value that should be protected. In the end it comes down to what kind of authenticity one intends to preserve. In the project, the experience values have been prioritized before the material values, which indicates that the authenticity from a material perspective may have decreased while the functional authenticity of the building has increased. The arguments that favour that the authenticity has been preserved are primarily based on the view that great emphasis has been placed on the preservation of the building's cultural-historical value. During the project one has also been inspired by the building's original drawings made by the building's architect Friedrich August Stüler, which indicates that the authenticity of the original building has been strengthened. The thesis ends with a final discussion where the result of the analysis is presented in relation to the theoretical framework of the thesis. This is done in order to answer the research questions. The main research question for the thesis has been; "How has the meaning of authenticity been presented in the final report, in the debate and by people who have worked with the reconstruction of Nationalmuseum?”.