Abstract: The U.S. Supreme Court's recent decision in Boumediene v. Bush elaborates a 'functional approach" to the selective application of constitutional limitations to U.S. government action outside U.S. sovereign territory. This functional approach provides the best fit, both descriptively and normatively, to the Court's modern case law. The decision repudiates the stance of the plurality in United States v. Verdugo-Urquidez, which sought to deny all constitutional rights to foreign nationals involuntarily subjected to U.S. action abroad. Important ambiguities remain in the articulation of the functional approach. One major question is whether and when foreign nationals who are not in U.S. custody (unlike the Boumediene petitioners) are also potentially eligible for constitutional protection. Another concerns how coarsely or finely the categories of foreign locations are drawn when the functional analysis is applied. The confirmation of the functional approach has significant consequences for U.S. citizens who travel abroad and for foreign nationals who travel here, as well as for foreign nationals who remain abroad. Although the Supreme Court did not rely on international law in its Boumediene decision, international human rights law may prove helpful in the future in determining whether limitations such as the First Amendment or the Takings Clause can practicably be given effect in foreign countries.