Crime is usually territorial. It is a matter of the law of the place where it occurs. Nevertheless, a surprising number of American criminal laws apply outside of the United States. Application is generally a question of legislative intent, expressed or implied. In either case, it most often involves crimes committed aboard a ship or airplane, crimes condemned by international treaty, crimes committed against government employees or property, or crimes that have an impact in this country even if planned or committed in part elsewhere. Although the crimes may be many, so are the obstacles to their enforcement. For both practical and diplomatic reasons, criminal investigations within another country require the acquiescence, consent, or preferably the assistance, of the authorities of the host country. The United States has mutual legal assistance treaties with several countries designed to formalise such co-operative law enforcement assistance. Searches and interrogations carried out on our behalf by foreign officials, certainly if they involve Americans, must be conducted within the confines of the Fourth and Fifth Amendments. And the Sixth Amendment imposes limits upon the use in American criminal trials of depositions taken abroad. Our recently negotiated extradition treaties address some of the features of our earlier agreements which complicate extradition for extraterritorial offences, i.e., dual criminality requirements and exemptions on the basis of nationality or political offences. To further facilitate the prosecution of federal crimes with extraterritorial application Congress has enacted special venue, statute of limitations, and evidentiary statutes.