In response to the terrorist attacks of 9/11, the USA PATRIOT Act greatly expanded the ability of the Federal Bureau of Investigation to use National Security Letters (NSLs) in investigations and the contexts in which they could be used by relaxing the standards under which NSLs could be employed. NSLs allow investigators to acquire a significant amount of information about the subject of an investigation, and the range and frequency of uses of NSLs has risen dramatically since the passage of the USA PATRIOT Act. An outstanding question with NSLs is whether they are being used in a manner that violates the First and Fourth Amendments of the Constitution. This article explores the relationships between NSLs, the USA PATRIOT Act, and the Constitution. The paper reviews the legislative history of NSLs, synthesizes the First and Fourth Amendment issues and key judicial decisions related to NSLs, and analyzes the extent to which the USA PATRIOT Act Improvement and Reauthorization Act of 2005 adequately addresses challenges to the constitutionality of NSLs. The paper concludes with an examination of NSLs within the greater information policy context in the United States, as the expanded usage of NSLs--even if entirely constitutional--raises the possibility of chilling the freedom of expression.