States have increasingly entered into treaties for mutual legal assistance in criminal matters. These treaties tend to be completed bilaterally. Typically under such treaties each party is required to establish a "competent authority" through which assistance should be directed, and the parties provide mutual assistance regarding the taking of evidence from witnesses, conducting searches and seizures, serving documents, and supplying documents and records. As with extradition, there are grounds on which a requested State may refuse to comply with a request. Regarding the admissibility of evidence obtained from abroad, none of the treaties provide means for ensuring that the evidence obtained in the requesting State will not encounter objections to its admissibility in the requesting State. A number of treaties do provide for the requesting State to outline any procedure they require to be followed, but no mandatory instructions as to procedure are required to accompany the request, thereby allowing variations between different requests and presumably, therefore, different standards of admissibility. This article discusses the interpretation of the provisions of mutual legal assistance treaties by U.S. courts. The discussion encompasses mutual legal assistance and alternative methods of obtaining evidence and the challenging of evidence obtained under treaty. A discussion of evidence obtained outside formal mutual legal assistance arrangements focuses on unlawful extra- territorial evidence gathering, collusive action between States in irregular evidence gathering, the unilateral passing of evidence, evidence that turns up unsolicited, "process- laundering" evidence obtained abroad, and problems of "specialty" in relation to evidence. The authors recommend that evidence obtained in a foreign jurisdiction and presented domestically satisfy a two-fold test: would such evidence, if obtained in this manner domestically, be admissible; and would such evidence, obtained in this manner, be admissible in the State in which it was obtained. 134 footnotes