Recent decades have seen increasing levels of political polarization. Whereas the literature has commonly distinguished between ideological and affective polarization, this chapter proposes that a third core pillar should be added to this taxonomy because citizens are divided not only in their attitudes and their feelings toward each other, but also in their factual perceptions of reality. For example, the vast majority (84%) of American citizens who identify as a Democrat accept the scientific consensus on anthropogenic climate change, but less than half (43%) of Republicans share this position. Similar partisan differences exist in factual beliefs about key issues such as the size of the immigrant population and the level of income inequality. Nearly any political attitude is likely to be accompanied by at least some factual assumptions, even if citizens are not quite sure about these beliefs or when they are not even consciously aware of them. This chapter argues that factual belief polarization may be viewed both as a consequence and a potential cause of other types of polarization. Although the evidence for the latter is still limited and inconclusive, it is easy to imagine how partisan divides in factual perceptions could fuel ideological disagreements and political hostility.