Over the past four decades, the Latino population of the United States was transformed from a small, ethnically segmented population of Mexicans in the southwest, Puerto Ricans in New York, and Cubans in Miami into a large national population dominated by Mexicans, Central Americans, and South Americans. This transformation occurred through mass immigration, much of it undocumented, to the point where large fractions of non-Caribbean Hispanics lack legal protections and rights in the United States. Rising illegality is critical to understanding the disadvantaged status of Latinos today. The unauthorized population began to grow after avenues for legal entry were curtailed in 1965. The consequent rise in undocumented migration enabled political and bureaucratic entrepreneurs to frame Latino migration as a grave threat to the nation, leading to a rising frequency of negative framings in the media, a growing conservative reaction, and increasingly restrictive immigration and border policies that generated more apprehensions. Rising apprehensions, in turn, further enflamed the conservative reaction to produce even harsher enforcement and more still more apprehensions, yielding a self-feeding cycle in which apprehensions kept rising even though undocumented inflows had stabilized. The consequent militarization of the border had the perverse effect of reducing rates of out-migration rather than inhibiting in-migration, leading to a sharp rise in net undocumented population and rapid growth of the undocumented population. As a result, a majority of Mexican, Central American, and South American immigrants are presently undocumented at a time when unauthorized migrants are subject to increasing sanctions from authorities and the public, yielding downward pressure on the status and well-being of Latinos in the United States.