As Eric Hobsbawm recounts in his classic work, The Age of Empire: 1875–1914, the final decades of the nineteenth century and the initial decades of the twentieth century were years of enormous change and activity across the globe. It was the apogee of imperialism for the West; mass, or at least more broadly based, democracy emerged in many countries; total wealth increased dramatically; technological changes greatly reduced travel times and facilitated rapid, even instantaneous, communication between states and continents, which, in turn, allowed the spread of mass culture in a way the world had never seen before. At the center of these events were the great powers of Europe—in particular Britain, France, Germany, Russia, and Austria-Hungary—and the United States. Indeed, the interaction between Europe's great powers and the United States drove much of the political, economic, cultural, and technological ferment that culminated in the First World War. No American played a more important role in this process than Theodore Roosevelt, and this special issue is devoted to exploring key facets of TR's, and by extension his country's, relationship with Europe.