This chapter examines how the processes of social modernization—economic development and political development—affect the political cleavage structure of a nation and consequently the structure of political and electoral competition. We first examine whether modernization lessens ideological conflict in terms of Left-Right polarization. Using empirical data from the Comparative Study of Electoral Systems (CSES) we find that most citizens in most nations have a Left-Right position. We argue that these Left-Right positions reflect the salient issues in the nation and the issues salient to each individual. Significantly, advanced industrial democracies display a lower level of ideological polarization than developing democracies. We also examine the social bases of these Left-Right orientations, demonstrating that existing social cleavages, levels of modernization, and historical patterns in the party system shape the correlates of Left-Right. The chapter also analyzes the impact of modernization on partisan attachments. Modernization initially strengthens party ties, but further development in advanced industrial begins a process of partisan dealignment. These findings illustrate how social modernization broadly shapes electoral processes in contemporary democracies. The intense Left-Right divisions in new, less affluent democracies, as well as lower levels of party attachments, may create highly divisive electoral politics and produce centrifugal pressures in the political system. In contrast, political cleavages are more moderate in advanced industrial democracies, and reflect a mix of historical issue concerns and the new political controversies facing these nations. In summary, as democracy develops the nature of the electorate systematically changes in both the level of political division and the sources of division.