How do undocumented migrants become activists? How can we make sense of the fact that undocumented activists are powerful, resourceful, and hyper-visible yet vulnerable, precarious, and relatively invisible all at once? How do the personal and collective activist trajectories of undocumented migrants diverge and converge on both sides of the Atlantic? And why do the actions they organize from the margins hit politics-as-usual in its very core? These are the complex and provocative questions that Thomas Swerts poses in Citizen X. Based on years of immersion as an ally in Chicago and Brussels, this transatlantic ethnography offers an unprecedented inside look into the birth, growth, and demise of undocumented-led organizations. In cities on opposite sides of the Atlantic, migrants struggled to create a space from where they could speak their own truth as lived experiences of illegality reinforced their feelings of powerlessness. But as Swerts reveals, under certain conditions, vulnerabilities can be developed into collective strengths. By openly sharing their stories, contesting restrictions on mobility, and publicly expressing their emotions, undocumented activists expose the hidden reality of 'Citizen X'. At a time when legal avenues for migration are becoming narrower and undocumented migrants are increasingly criminalized and stigmatized, Citizen X discloses the fundamental inequalities and social injustices that result from unevenly distributing legal status in the world while highlighting the renewed meaning that non-citizens are giving to citizenship from the ground up.