The safety of air travel, particularly after the terrorist attacks of September 11, 2001, has been an important priority for the U.S. government. The Aviation and Transportation Security Act of 2001 created the Transportation Security Administration (TSA) and charged it with ensuring the security of all modes of transportation, including civil aviation. The TSA is responsible for prescreening all potential commercial airline travelers before they board an aircraft. Pursuant to this responsibility, TSA uses the "No Fly" list to identify individuals who pose a threat to aviation safety. Persons attempting to board an aircraft who are matched to an identity on the No Fly list are not allowed to board. Although the current number and identity of persons on the No Fly list is not a matter of public record, some recent news reports claim that more than 80,000 people are currently on the list. However, some persons have claimed that their alleged placement on the list was the result of an erroneous determination by the government that they posed a national security threat. In some cases, it has been reported that persons have been prevented from boarding an aircraft because they were mistakenly believed to be on the No Fly list, sometimes on account of having a name similar to another person who was actually on the list. The Department of Homeland Security (DHS) operates a redress process for travelers who wish to contest their right to board an aircraft, but this procedure has been challenged in federal court as violating the Fifth Amendment right to due process. After an adverse ruling in a federal district court case in 2014, the executive branch revised the process. This report provides an overview of the operation of the government's watchlists, examines some of the legal issues implicated by challenges to the No Fly list, and describes recent case law on the matter. The National Counterterrorism Center (NCTC) serves as the central information bank for the U.S. government on "known and suspected terrorists and international terrorist groups." It is the government's principal organization for "analyzing and integrating" intelligence concerning terrorism and counterterrorism. The NCTC maintains the Terrorist Identities Datamart Environment (TIDE), the central repository of the U.S. government containing derogatory information about suspected international terrorists. Based on evaluations of intelligence information, agencies in the intelligence community (IC) nominate individuals known or suspected to be international terrorists and forward the names to the NCTC. Using a non-exclusive list of possible factors, the NCTC determines if each name merits inclusion on the list. As of June 30, 2016, according to the NCTC, about 1.5 million persons were included in TIDE, and about 15,000 were U.S. persons (citizens and lawful permanent residents). TIDE contains all of the government's information regarding persons "known or appropriately suspected to be or to have been involved in activities constituting, in preparation for, in aid of, or related to terrorism (with the exception of purely domestic terrorism information)." Due to the national security importance of this information, the contents of the database are classified. The NCTC "exports" an unclassified subset of the data, including biometric and biographic identifiers, to the Terrorist Screening Center (TSC), which, in turn, operates the Terrorist Screening Database (TSDB). In contrast to TIDE (operated by NCTC), the TSDB (operated by TSC) does not include "derogatory intelligence information." Instead, it consists of "sensitive but unclassified terrorist identity information consisting of biographic identifying information such as name or date of birth or biometric information such as photographs, iris scans, and fingerprints." Established pursuant to Homeland Security Presidential Directive 6, the TSC is managed by the Federal Bureau of Investigation (FBI) and receives support from various federal agencies. The information in the TSDB is obtained from two primary sources. First, as mentioned above, TIDE provides information on the identity of suspected international terrorists. Second, the FBI's Automated Case Support System (ACSS) provides additional information on suspected domestic terrorists directly to the TSC. Whether receiving information from TIDE or ACSS, the TSC will review each file to ensure that it satisfies the government's watchlist standards before adding the name to the TSDB. The information received by TSC must satisfy two requirements to merit inclusion on the TSDB. First, the "biographic information associated with a nomination must contain sufficient identifying data so that a person being screened can be matched or disassociated from a watchlisted terrorist." Second, the "facts and circumstances" must "meet the reasonable suspicion standard of review." This means "articulable facts which, taken together with rational inferences, reasonably warrant the determination that an individual is known or suspected to be or has been engaged in conduct constituting, in preparation for, in aid of, or related to terrorism and terrorist activities." This standard was not mandated by statute, but was "adopted by internal Executive Branch policy and practice." In addition, a 2014 district court opinion stated that there was a "secret exception to the reasonable suspicion standard," but the "nature of the exception and the reasons ... for nomination are claimed to be state secrets." As mentioned above, in contrast to the classified contents of TIDE, the TSDB contains sensitive, but not classified, information about the identity of suspected terrorists. The unclassified nature of the list permits a broad range of federal, state, and local organizations to access the data. Accordingly, the TSC provides various frontline screening agencies with subsets of the TSDB for use in combating terrorism. These watchlists are tailored in accordance with the agency's mission(s) and statutory authorities. For the purposes of monitoring flights, TSA receives two such lists: the No Fly list and the Selectee list. People on the first are prohibited from boarding an American airline or any flight that comes in contact with U.S. territory or airspace. Those on the second are subject to enhanced screening procedures when they attempt to do so. The No Fly and Selectee lists have their own substantive requirements for inclusion, which executive officials have stated are more stringent than the reasonable suspicion standard for placement on the TSDB. In order to qualify for inclusion on the No Fly or Selectee lists, a nomination must "establish[] a reasonable suspicion that the individual meets additional heightened derogatory criteria that goes above and beyond the criteria required for inclusion in the broader TSDB." When a person is placed on the list, they will not receive notice; instead, they will simply be denied boarding or subjected to enhanced screening procedures if they attempt to board a plane. However, in a departure from the traditional requirements for inclusion on the No Fly list, after the failed terrorist attack of the so called "underwear bomber," who attempted to destroy a commercial plane traveling from Amsterdam to Detroit on Christmas Day 2009, the NCTC and TSC were ordered, following an interagency meeting, to add a number of individuals from the TIDE database to the No Fly list. This included a number of "individuals without any information indicating a personal involvement in terrorism." Accordingly, a number of individuals were placed on the No Fly list who may not have met the normal standards for inclusion. Subsequently, TSC, in coordination with the FBI and other intelligence agencies, conducted a review of all the individuals who had been upgraded. This review was completed more than two years after the original upgrading. A 2014 Department of Justice (DOJ) Office of Inspector General audit expressed "concerns about the TSC's ability to ensure that all watchlist records that were modified as a result of the attempted attack were reviewed and returned to the appropriate individualized status." The precise guidelines and particular factors the government relies on to place individuals on terrorist watchlists are not made public. The criteria for placement on the No Fly list, as well as whether a person is on the No Fly list, are considered "Sensitive Security Information" (SSI) and have not been publicly released by the federal government. Initially, TSA required aircraft operators to screen passengers by matching data against the No Fly and Selectee lists. Following the release of the 9/11 Commission Report , the Intelligence Reform and Terrorism Prevention Act of 2004 altered this arrangement by requiring TSA to conduct the matching itself. The act also requires aircraft operators to provide passenger information to TSA for prescreening purposes. TSA issued the Secure Flight Final Rule on October 28, 2008, implementing the act's requirements. Under the program, TSA requires aircraft operators to collect Secure Flight Passenger Data (SFPD) from passengers and provide it to TSA. SFPD includes passengers' full name, date of birth, gender, and can also include certain non-personal information, such as itinerary and a travel record number. The information is collected when a potential passenger makes a flight reservation. This information must be provided to TSA about 72 hours prior to the flight. For reservations that occur after this deadline, aircraft operators must provide the SFPD as soon as possible. TSA then matches the data with the No Fly, Selectee, and Centers for Disease and Control and Prevention (CDC) Do Not Board list. The potential matches are compared to the TSA Cleared List, which contains the identities of individuals who have been cleared through the DHS redress process. TSA then performs manual reviews of potential matches to ensure individuals are included on the No Fly and Selectee lists, which may include the consultation of other databases. Following this process, TSA will prohibit certain passengers from receiving a boarding pass. If such passengers arrive at the airport, air carriers may send updated passenger information to TSA for rematching or may call Secure Flight to resolve an issue. This process may involve contact between Secure Flight and the Terrorist Screening Center to confirm or eliminate a match. At the conclusion of this procedure, a passenger will either be cleared to fly, designated as a selectee for enhanced screening, or barred from boarding the aircraft.