How far does U.S. law reach beyond U.S. borders? In principle, Congress could extend its laws as far as it likes, but Congress often fails to make its intentions clear. Many statutes do not specify their geographic scope, instead using general terms that have no inherent limit. Federal courts have long employed interpretive rules, or canons, to guide their construction of such statutes. The canon most commonly cited is the presumption against extraterritoriality, which states that “legislation of Congress, unless a contrary intent appears, is meant to apply only within the territorial jurisdiction of the United States.” The simplicity of this language masks difficult questions. Does the “territorial jurisdiction of the United States” include only territory within the national boundaries, or does it extend to territory outside those boundaries but within U.S. control? How should the presumption apply to actions taken abroad that cause effects within U.S. territory, however defined? What implications does the presumption have for situations within U.S. territory but also within the jurisdiction of another country, such as foreign ships in a U.S. port? When the presumption does apply, what evidence of “contrary intent” is necessary to overcome it?