This book provides fundamental insights into the pontificate of Gregory IX through the little-studied Life of Pope Gregory IX and considering several other sources. It examines the Life’s genesis and transmission, its relation to the medieval papal historiography, its programmatic orientation towards the governance of the Papal States, the significance of the two excommunications of Frederick II in the mirror of the Life, and its stylisation of the papal protagonist and imperial antagonists. The Life served the papacy primarily as a document of legitimation and defence in the crisis years of 1239/40, and also as a source of political, financial, legal, and liturgical responses for the posterity of the Roman Curia. Gregory IX left a longer-lasting mark on the papal sanctity, the territorial politics, and the eschatological thinking of the Roman Curia than recognized by previous scholarship.