Early electoral research in the United States discovered the most important concept in the study of political behavior: party identification. Party identification is a long-term, affective attachment to one’s preferred political party. Cross-national research has found that these party identities are a potent cue in guiding the attitudes and behavior of the average person. Partisans tend to repeatedly support their preferred party, even when the candidates and the issues change. Party ties mobilize people to vote to support their party, and to work for the party during the campaign. And, given the limited information most people have about complex political issues, party ties provide a cue to what positions one should support. This review describes how the level of partisanship among contemporary publics varies across nations and across time, and how these patterns have significant implications for democracies today.