Party polarization in the House of Representatives has increased recently. Explaining this development has been difficult given current interpretations of American elections. The dominant framework for interpreting elections has been to see them as candidate-centered or individualistic. This book explains the emergence of party polarization by focusing on how the constituencies of House districts affect partisan outcomes and the subsequent voting behavior of House members. The analysis is premised on the simple argument that members are elected from districts, and an explanation of polarization must begin with districts. The origins of polarization lie in the realignment of the electoral bases of the parties, and the shifting demographic composition of America. The analysis will focus primarily on changes since the 1960s.