An estimated 7 percent of people in Mexico were not registered with the government at birth and thus lack official record of their name, age, parentage, and citizenship. Without a birth certificate, unregistered Mexican children lack access to education, health care, and basic social services, while unregistered adults face significant economic and civic-integration challenges. Both groups are more vulnerable to being trafficked, exploited, or recruited into criminal groups. This article provides a primer on this important issue, with insight into the experiences of unregistered, unauthorized Mexican immigrants in the United States.