Cardinal Ippolito II d'Este had a role of primary importance in shaping the diplomatic connections and the political destiny of the duchy of Ferrara in the sixteenth century. However, scholarship has traditionally overlooked this figure, seeing him only as the archetypal worldly cardinal. The existence of a large number of works dedicated to the Cardinal's patronage of prominent artists and writers, in both France and Italy, emphasises, by contrast, the lack of an extensive study on the particular historical conditions that determined the nature of the Cardinal's power. In order to highlight the peculiarity of the Cardinal's power, this thesis seeks to connect existing scholarship on Ippolito d'Este with primary sources from the Archive of Modena and with more recent studies on the relationship between the Vatican and the Italian and European players, on the organisation of curial life, and on the strategies of survival adopted by small Italian rulers. The figure of Ippolito d'Este is examined through the analysis of different episodes - singularly known to historiography but examined neither together in a continuous narrative nor with a focus on the important role that Ippolito played in them - in order to highlight the interplay of powers that were simultaneously represented by the Cardinal. The conflicted nature of Ippolito's power, which owed loyalty to the Este, the King of France, and the Pope, was, I argue, a problematic element in the performance of those powers themselves. Furthermore, the Cardinal's strong involvement with French politics in Italy heavily interfered with his family's policy and may be seen as the main reason for the Este's long-term failure to establish themselves as a successful ecclesiastical dynasty.