In the early hours of a cold November morning in 2013, six people formed a circle and sat down in the middle of a suburban Chicago street. Working quickly, they attached themselves to each other inside of pipes that enclosed their forearms; two other groups of three lay down on the street and encircled the front wheels of a bus. As dozens of people gathered around, they settled in for what would be an hours-long standoff with Immigration and Customs Enforcement (ICE) agents in front of the immigrant detention center in Broadview, Illinois. The purpose of the protest was to block the bus, which was departing from Broadview to ferry immigrants to O’Hare airport and their subsequent deportations. Dozens of supporters led chants and gave short speeches condemning the system of mass deportation that had ensnared the detainees in Broadview and led to record-breaking deportation rates since 1996. The detainees seated in the blocked bus peered out of the windows and, at one point, stamped on the floor and walls to show their solidarity with the protesters. All six activists in the circle were arrested that day; five of them were undocumented, and the sixth was me, Amalia Pallares (Figure 1). Later, people would ask me why, as a middle-aged professor of political science, I would get arrested on purpose, putting my body and freedom on the line to demand immigration reform. A better question would be why my undocumented friends and colleagues would do it. After all, for me, the potential consequences include misdemeanor charges of mob action or riot; for them, they include deportation and permanent banishment from their US homes and families. Why would they risk bringing themselves to the attention of ICE agents? Or, put another way, what kind of politics would drive undocumented people to risk it all to mobilize against US immigration law? And how might being undocumented itself inform a specific kind of political struggle? The articles in this special issue draw on extended ethnographic research to explore how undocumented people, legally excluded from political participation in the nation, engage in a kind of “impossible activism,” transforming political discourse and policy even as their presence is negated by law (Pallares 2014). Their actions advance our understanding of practices of “substantive citizenship,” such as the political takeover of public spaces by noncitizens or otherwise politically marginalized populations (eg, Flores 1997; Pallares 2014; Rosaldo 1997) and illuminate dynamic sites of struggle that are continually coproduced by state agents and those they seek to govern. Indeed, as the articles here attest, it is precisely this dynamism—the ever-evolving nature of political understandings and strategies of protest—that best characterizes the contemporary movement of undocumented workers and youth. Because the state is not unified and immigration policies are in constant flux, undocumented organizers must continually adapt their strategies to changing conditions, regulations, and levels of government. At times, this means deploying tactics that tap into existing norms and political categories, and at times, this means deploying tactics that challenge them. As they mobilize, undocumented activists draw on a legacy of political protest in Latin America and the United States, and they interconnect with other movements, such as the Civil Rights, Gay Rights, and Black Lives Matter movements, to build a “politics of motion” that is constantly regenerated through struggle (Pallares 2014; Unzueta and Seif 2013).