A phenomenon as complex as national-level turnout cannot be explained with one simple variable or even one theoretical framework. In fact, there are several potential explanations for the differential turnout levels between the United States and other industrialized democracies. This book argues that the institutional explanation for the turnout problem in U.S. elections provides the most comprehensive explanation. But before discussing the institutional explanation in greater detail, I shall examine the various potential explanations for the turnout problem in the United States.