Polarization has been studied in three different forms: on a social, group, and individual level. This entry first focuses on the undisputed phenomenon of elite polarization (i.e., increasing adherence of policy positions among the elites) and also outlines different approaches to assessing mass polarization (i.e., political positions among the public, differences between issue preferences of supporters of the major parties, and partisan sorting or the alignment of partisan and ideological identities). Second, group polarization and attitude polarization, most often studied in social psychology, are outlined, with the focus on how both have been empirically tested. Group polarization typically refers to the tendency for groups to make decisions that are more extreme than the initial inclination of individual members, whereas attitude polarization is usually detected when an individual's own attitude following media exposure or political talk becomes more extreme but in the same direction as their pretest attitude.