Part I. Polarization among Voters and Activists: 1. The new American electorate: partisan, sorted, and polarized Alan I. Abramowitz; 2. Redistricting and polarization Micah Altman and Michael McDonald; 3. Party activists, interest groups, and polarization in American politics David Karol; 4. Authoritarianism and polarization in American politics, still? Marc J. Hetherington and Jonathan Weiler; 5. Party sorting: the foundation of polarized politics Samuel J. Abrams and Morris P. Fiorina; Part II. Polarization in National Institutions: 6. Presidential-congressional relations in an era of polarized parties and a sixty-vote Senate Jon R. Bond, Richard Fleisher and Jeffrey E. Cohen; 7. Party warriors: the ugly side of party polarization in Congress Sean M. Theriault; 8. The sources and consequences of polarization in the US Supreme Court Brandon L. Bartels; Part III. Polarization in the States: 9. Polarization in American state legislatures Boris Shor; 10. The costs of party reform: two states' experiences Seth E. Masket; 11. The policy consequences of party polarization: evidence from the American States Elizabeth Rigby and Gerald C. Wright; Part IV. Polarization in the Media: 12. Partisan media and electoral polarization in 2012: evidence from the American National Election Study Gary C. Jacobson; 13. News as a casualty: district polarization and media coverage of US House campaigns Danny Hayes and Jennifer L. Lawless; 14. More a symptom than a cause: polarization and partisan news media in America Kevin Arceneaux and Martin Johnson; 15. The polarizing effects of partisan and mainstream news Natalie Jomini Stroud and Alexander Curry; Part V. Implications and Conclusions: 16. Congressional polarization and its connection to income equality: an update Adam Bonica, Nolan McCarty, Keith T. Poole and Howard Rosenthal; 17. The sources and impact of political polarization James A. Thurber and Antoine Yoshinaka.