This volume collects and analyzes eleven country case studies of polarized polities that are, or had been, electoral democracies, identifying the common and differing causal mechanisms that lead to different outcomes for democracy when a society experiences polarization. In this introduction, we discuss our goals for the volume, the comparative logic we apply to the cases, our overall methodological approach, and the concepts that ground the analyses. The goal of this volume is to explore pernicious polarization, i.e., when and how a society divides into mutually distrustful “us vs. them” blocs, which endangers democracy. Accordingly, we discuss the effects of such polarization on democracies, and start building a foundation for remedies. In this introductory article, we highlight and explain the inherently political and relational aspects of polarization in general and pernicious polarization in particular, present the concept of formative rifts, and discuss how opposition strategies should be part of an explanation of severe polarization.