Limitations on involuntary government detention lie at the core of our constitutional scheme, with the U.S. Supreme Court long having recognized that the Due Process Clauses of the Fifth and Fourteenth Amendments impose fundamental substantive and procedural protections against unjustified detention. Yet nearly 250 years after the adoption of the Bill of Rights, in a country peopled by immigrants, these most basic of constitutional protections remain unresolved for one group: noncitizens detained on suspicion of civil offenses involving U.S. immigration law.