While previously polarization was primarily seen only in issue-based terms, a new type of division has emerged in the mass public in recent years: Ordinary Americans increasingly dislike and distrust those from the other party. Democrats and Republicans both say that the other party’s members are hypocritical, selfish, and closed-minded, and they are unwilling to socialize across party lines. This phenomenon of animosity between the parties is known as affective polarization. We trace its origins to the power of partisanship as a social identity, and explain the factors that intensify partisan animus. We also explore the consequences of affective polarization, highlighting how partisan affect influences attitudes and behaviors well outside the political sphere. Finally, we discuss strategies that might mitigate partisan discord and conclude with suggestions for future work.