This article compares the dynamics of polarization in the eleven case studies analyzed in this special issue to draw conclusions about antecedents of severe political and societal polarization, the characteristics and mechanisms of such polarization, and consequences of severe polarization for democracy. We find that the emergence of pernicious polarization (when a society is split into mutually distrustful “Us vs. Them” camps) is not attributable to any specific underlying social or political cleavage nor any particular institutional make-up. Instead, pernicious polarization arises when political entrepreneurs pursue their political objectives by using polarizing strategies, such as mobilizing voters with divisive, demonizing discourse and exploiting existing grievances, and opposing political elites then reciprocate with similarly polarizing tactics or fail to develop effective nonpolarizing responses. We explain how the political construction of polarization around “formative rifts” (social or political rifts that arise during the fundamental formation/reformation of a nation-state), the relative capacity of opposing political blocs to mobilize voters versus relying on mechanisms such as courts or the military to constrain the executive, and the strategic and ideological aims of the polarizing actors contribute to the emergence of its pernicious form. We analyze the consequences for democracy and conclude with reflections on how to combat pernicious polarization.