This book addresses the new elements and legal issues pertaining to immigration enforcement in the United States. Topics include the scope of prosecutorial discretion in immigration enforcement; a look at whether administrative amnesty harms our efforts to gain and maintain operational control of the borders; defining and quantifying the criminal alien population and enforcement statistics; the various authorities governing immigration detainers and key legal issues; authority of the state and local police to enforce federal immigration law; the Supreme Court's ruling in Arizona v. United States and the implications for immigration enforcement activity by states and localities; immigration-related worksite enforcement performance measures; and border security and immigration enforcement between ports of entry.