George Washington warned in his farewell address that “the spirit of party ... is inseparable from our nature, having its root in the strongest passions of the human mind.” Indeed, while many factors influence how citizens judge, reason, and make decisions about politics, parties and partisanship play an extraordinarily central role in political cognition. Party and partisanship color how individuals understand the political world in two broad ways. Partisan stereotypes, or how party labels call to mind a host of attributes about people and constituent groups, play an important role in cognition. Second, perhaps even more pronounced in a hyperpolarized political world, is the way in which party influences cognition through partisan identity, or one’s own attachment (or lack thereof) to one of the parties. This connects a party and co-partisans with one’s own self-concept and facilitates an us-versus-them mentality when making political judgments and decisions. Both cognitive pathways are often simultaneously operating and interacting with each other. While we can think about the role of party in terms of stereotypes or identities, the impact of partisanship on actual cognition often involves both, and it can have varied implications for the quality of political decision making. Because partisanship is central to the political world, particularly in democracies, it has been the subject of a variety of lines of inquiry attempting to explain its impact on voters’ decisions.