Scholars of public opinion attempt to describe and account for the politically relevant preferences and beliefs of ordinary citizens, and to assess the political impact of those preferences and beliefs. The development and proliferation of systematic opinion surveys transformed the study of public opinion in the last half of the twentieth century. Analysts have used survey data to examine the social bases, structure, and sophistication of political opinions in elaborate detail, often with an eye toward evaluating the democratic competence of citizens. They have generally been more successful in characterizing and accounting for public opinion than in specifying its role in the broader political process. Efforts to specify the political causes and consequences of public opinion will no doubt be facilitated by the continuing accumulation of survey data over time and space, but will also require scholars of public opinion to make more ingenious and eclectic use of other relevant evidence in combination with survey data.