Introduction: Karl Häupl is considered one of the most famous European dentists of the 20th century. The aim of this article is to trace the life and work of the Austrian university lecturer and to evaluate his professional contributions. An additional focus is put on Häupl's role in the Third Reich. Material and Methods: The methodological basis of the study is the evaluation of sources from various German and Austrian archives (some of which have been evaluated for the first time) and a critical re-analysis of the relevant research literature. Results: The analysis demonstrates that Häupl was one of the most influential and effective university lecturers in dentistry and, in particular, one of the pioneers of functional orthodontics; this is fully in line with the contempo-rary assessment. In contrast, the classification of his role in the "Third Reich" is clearly discrepant: Although Häupl's relationship to National Socialism was not addressed for decades, the archival sources provide clear evidence of political entanglement. Discussion and Conclusion: The analysis of the relevant sources leads to the conclusion that in the Third Reich Häupl was politically true to the line. He did not only join the National Socialist Party, but also enjoyed the backing and support of the major Nazi networks in his chair applications (German University Prague, University Berlin), honours, and further initiatives. Keywords: NSDAP, National Socialism, functional orthodontics, history of dentistry, periodontology