This introductory chapter examines arguments for and against adopting the All-Affected Principle (AAP) as a criterion for democratic inclusion, and the alternatives. For many, the attraction of the AAP lies in its straightforward simplicity: If you are affected by a collective decision, you should be able to influence it. Yet there remains sharp disagreement among scholars of democracy about how to best formulate the AAP and the circumstances in which it applies. Surveying the literature, we argue that appeals to the AAP will vary according to: (1) organizational scope; (2) decision-making context; (3) kinds of influence; (4) how influence is allocated; (5) the definition of “affectedness”; and (6) the stringency of any participatory requirements. Whether the AAP is consistent with existing arrangements, or requires a more radical redrawing of democratic boundaries, is a question on which opinions may differ significantly. We conclude by discussing the trade-offs between more versus less ambitious versions of the AAP, the implications for addressing pressing governance challenges, and the future of the democratic project more generally.