Unionists part ways when it comes to the practical meaning of solidarity with workers who lack work permits. To some union members, undocumented migrant workers ought to be included in the bonds of union solidarity by virtue of being workers. To others, undocumented migrant workers are primarily illegal and unfair competitors undermining existing institutions of solidarity. In this controversial context, six union centers for undocumented migrant workers called MigrAr (German "Migration & Arbeit", English "migration & labor") have been established by labor activists since 2008. Building on an activist ethnography in the MigrAr center in Berlin from 2011 to 2015, this investigation charts the challenges of this center to bring unions to adopt a more proactive and inclusive approach towards illegalized migrant workers. In the analysis, the research critically engages Jürgen Habermas's concept of solidarity and its premise of symmetrical relationships. To account for the uneven constellation of support between MigrAr activists and undocumented migrant workers, the study advances an alternative understanding of union solidarity. Addressing labor activists and scholars in the social sciences, this book gives detailed insights in a controversial field of union activism in Germany. This study contributes to a rethinking of union solidarity under current dynamics of migration.