Each year, tens of thousands of undocumented youngsters leave American high schools to embark upon uncertain futures. But until now, very little has been known about the ways in which these young people come of age and how legal barriers shape their adolescent and adult trajectories. Drawing on extensive fieldwork in Greater Los Angeles, and 150 life history interviews with 20-34 year old, Mexican-origin, undocumented young adults, this study finds that conflicting and contradictory laws uncomfortably position undocumented immigrant youth between spaces of belonging and illegality. As laws begin to narrowly circumscribe their everyday lives they must learn to be illegal. Ultimately, through their transition to illegality, immigration status emerges as a master status.