Chapter 13. Undocumented Immigration and Human Trafficking Border and interior enforcement legal penalties and the market for smugglers distinguish undocumented migration from legal migration and introduce a number of challenges for migrants employers law enforcement and policymakers. This chapter reviews the theoretical models and empirical evidence on the determinants of undocumented migration and its nature and prevalence around the world. Stricter enforcement vis à vis undocumented migration leads to unintended consequences such as reduced circular migration and greater demand for smugglers. Although recent trends are generally toward stricter enforcement and more punitive sanctions for undocumented migrants and smugglers there have been periods in recent history when undocumented immigration has been viewed with benign neglect. We review the costs and benefits of undocumented migration from a number of angles including destination and origin countries firms consumers taxpayers native born workers and migrants themselves. We discuss the long run impact of undocumented status on immigrants and their families including the detrimental effect on integration and the limited extent of access to public benefits such as health care and public pensions. We consider the tradeoffs of regularization programs which may have economic benefits in the short run but may lead to increased undocumented immigration in the long run. Finally we turn to human trafficking which involves coercion and review models and empirical evidence on human trafficking and policy responses.