The Duke of Boulogne is a title historically associated with the region of Boulogne-sur-Mer in northern France. This title was often held by noble families and played a significant role in the feudal system. The duke's authority included governance over the territory and its inhabitants, as well as military responsibilities. The title has connections to various historical figures, including the famous Duke of Boulogne from the House of Bourbon. The title has evolved over time, reflecting changes in political power and territorial control in France, particularly during the medieval and early modern periods.