A pdf is available by request to jmheyman@utep.edu . Article is in Spanish (translation of Núñez and Heyman 2007). In processes of entrapment, police and other state agencies impose significant risk to moving around, while people themselves exercise various forms of agency by both limiting themselves and covertly defying movement controls. Recent US immigration and border enforcement policy has entrapped undocumented immigrants, in particular on the United States side of the US-Mexico border region. We explore how to operationalize this "macro" pattern in ethnographic research, making the conceptually and methodologically significant point that political-legal forces are only one among many elements leading to entrapment and immobilization; other factors include transportation constraints, poor health, etc. The concept of "morality of risk" is also introduced to help us understand how and why trapped people would take severe risks to defy immigration policing. Three ethnographic cases are examined, noting the complex mix of movement and barriers found in them. We conclude with the significance of entrapment for applied and basic social science: first, for the analysis of spatial mobility, enclosure, and inequalities of movement; second, for public policy; and third, for the methods and ethics of researching trapped and hidden populations.