In deep disagreements, parties disagree about relatively fundamental underlying moral or epistemic principles and therefore see each other as less than fully rational or morally subpar. The chapter argues that deep disagreements lead to both cognitive and practical polarization, especially when they concern matters that are central to people’s social identities: deeply disagreeing parties will think less of each other and tend to treat each other worse. This, in turn, entrenches their disagreement even further, resulting in a vicious feedback loop. Support for the claims made here comes from both conceptual connections between deep disagreement and polarization as well as widely established empirical results in psychology and political science.