Cardinal Boso's papal biographies offer a unique insight into the history of the 12th century, which was shaped by schisms in the church and the struggle with the German Empire. Among his texts, the vita of Pope Alexander III stands out. (1159-11181), who not only experienced one of the longest and most important pontificates of the Middle Ages, but also faced Emperor Friedrich I Barbarossa in a decades-long conflict. Boso's contemporary biography was written against the background of this debate, which was still current at the time, and is by no means a neutral representation of history. Rather, the cardinal argued more, sometimes less subtly, in favor of his pope and was not above conscious misrepresentations. The present work approaches this important text from a source-critical perspective and comes to new insights into the course of the dispute between pope and emperor as well as the world view of the medieval curia.