Democracy means self-governance. But who gets to participate in this process, where, and by what means? The basic democratic proposal, that we govern ourselves, raises hard questions about boundaries. Democratic commitments often seem to collide with boundaries - yet the latter may be necessary for democratic practices. Around the world these issues take dramatic forms in arguments about citizenship, voting and inclusion, immigration, borders (and secession), and modes of political and cultural expression and conflict. We analyze these general issues with a focus on recent and ongoing politics in the United States.