One of the most striking features of modern economies is the uneven distribution of economic activity. Activities, such as production and employment, are unevenly distributed across space. Measuring geographic patterns of economic activities is difficult because doing so requires large amounts of disaggregate data. Information is needed about where individuals work and what sector they are employed in and this information must be available for highly disaggregated geographic units, such as local labor markets. Given these data requirements, many previous measures of economic geography fall short of capturing the theoretical concept of interest and most are available for only a single country. To address these limitations, I generate a continuous empirical measure of the geographic dispersion of sector-specific employment in more than ten countries using entropy indices. Measuring geographic concentration is an important first step to investigate how economic geography influences politics and policy in countries with various political institutions. I leave until Chapter 4 the technical details about the construction of the measure of economic geography. In this chapter, I explore the concept of economic geography: what it is, why does it vary, and how might it matter for politics?