If we were to look at a political map of the world in 1750, the most dominant configuration would not be nation states but empires. The same is true for a map of 1850, especially when we remember that most of the nation states of Europe at that time were also empires or aspiring empires. The categories of "empire" and "nation" were not mutually exclusive, and people around 1850 were more likely to consider the two as simultaneous and mutually beneficial rather than as diachronic and antagonistic. We would expect a map of 1950 to reflect nation-state hegemony, especially with the rise of the United Nations in 1945, but, even then, we would still be able to observe empires in operation around the world. Growing United States and Soviet power at that time could be framed as imperial paradigms in their own right. Empire has been a fundamental structure of world history for thousands of years, and this has continued to be the case in the last few hundred years, the so-called modern period. "By comparison," as historians Jane Burbank and Frederick Cooper argue in their Empires in World History (2010), "the nation-state appears as a blip on the historical horizon, a state form that emerged recently from under imperial skies and whose hold on the world’s political imagination may well prove partial or transitory." This chapter will discuss four related questions about the history of imperialism since about 1750, and the way this has been studied. The first and second might appear simple, but have engendered much heated debate in academic and public circles: "What are empires?" "How did they work?" The traditional approach described imperial expansion as a monolithic extension of core power, but scholarship of late has come to appreciate the ways in which imperial formation was incomplete, heterogeneous, and contingent upon local conditions. In that regard, the third question, "How did resistance matter?" is one of the most pressing in imperial studies today.