In recent years, state and federal laws have facilitated law enforcement's expanded use of deoxyribonucleic acid (DNA) for investigating and prosecuting crimes. These laws authorize compulsory collection of biological matter, which local law enforcement agencies send to the Federal Bureau of Investigation (FBI) for analysis. The FBI then stores unique DNA profiles in a national distributive database, through which law enforcement officials match individuals to crime scene evidence. Early laws authorized compulsory extraction of DNA only from people convicted for violent or sex-based felonies, such as murder, kidnapping, and offenses "related to sexual abuse"—crimes associated with historically high recidivism rates and for which police were likely to find evidence at crime scenes. However, in recent decades, new laws have greatly extended the scope of compulsory DNA collection, both by expanding the range of offenses triggering collection authority and, more recently, by authorizing compulsory collection from people who have been arrested but not convicted. Opponents of DNA databases suggest that DNA databases are "Orwellian" because of the amount of information about private citizens that they put into the control of the government. The most frequent criticism is that the programs violate the Fourth Amendment to the U.S. Constitution. Several federal courts have heard cases alleging that it is unconstitutional for an individual's pre- or post-trial release to be conditioned on DNA collection. Another Fourth Amendment argument, albeit a less litigated one, contends that it is unconstitutional to permit the use of databanked DNA profiles for purposes other than identifying a genetic match with a suspect. The Fourth Amendment protects individuals' privacy from unreasonable searches and seizures by the government. Federal courts have generally held that compulsory DNA collection from a person who has been convicted of a felony or other qualifying crime and placed under the supervision of the criminal justice system does not constitute an unreasonable search under the Fourth Amendment. These courts found that a convicted felon has a diminished expectation of privacy and that DNA profiling is a minimal intrusion into that privacy. Far fewer cases have given the federal courts an opportunity to decide whether DNA collection from arrestees is also constitutional. The two federal circuit courts of appeals to hear the question upheld the mandatory DNA profiling of indicted arrestees, but no federal court has assessed the constitutionality of profiling arrestees in the absence of a grand jury indictment or judicial finding of probable cause. Similarly, the courts have not yet had an opportunity to articulate the constitutional limits on how databanked DNA profiles may be used. This report traces the expansion of the statutory authorities for DNA databases and identifies emerging areas of consensus and discord among the federal courts over the Fourth Amendment consistency of compulsory DNA collection and the use of DNA databases. It also predicts additional Fourth Amendment issues that may come before both Congress and the federal courts in the near future. DNA is a complex molecule found in the nucleus and mitochondria of an organism's cells. It consists of two strands of nucleotides, the sequence of which contains the information that forms the basis of the human genetic code. The vast majority of human DNA is exactly the same, but small variations in the sequencing of the nucleotides create people's distinguishing characteristics. Only identical twins share the same DNA profile. With the help of DNA profiling technology, forensic scientists can examine different regions—or "loci"—of DNA to develop a DNA profile of the person from whom the DNA was extracted. Because forensic analysts examine a select group of loci, the resulting DNA profile may not necessarily be unique to that individual. However, advances in technology have enabled analysts to produce increasingly discriminating profiles. Today, the probability that two unrelated individuals would share a DNA profile derived from an uncontaminated sample of DNA from a cell's nucleus is estimated to be one in a billion at most. DNA profiles are often compared to fingerprints. As with fingerprints, law enforcement officers collect DNA samples from specific classes of individuals, such as prisoners. However, compulsory DNA collection generally entails blood or saliva samples rather than finger impressions, and DNA profiles can later match any of many types of biological matter obtained from crime scenes. For these reasons, DNA matching is considered a complement to, rather than merely a supplement for, fingerprint analysis in identifying criminal suspects. The FBI administers DNA storage and analysis for law enforcement agencies across the country. FBI analysts create DNA profiles by "decoding sequences of 'junk DNA.'" So-called "junk DNA" is the name for DNA loci that are "not presently recognized as being responsible for trait coding." Because junk DNA is not currently "associated with any known physical or medical characteristics," its use in forensic analysis prevents, at least for the time being, DNA profiles from containing private or sensitive information about the subject. Typically, a law enforcement agency's phlebotomist collects a blood or saliva sample from the subject pursuant to state or federal law. The sample may then be analyzed and converted into a DNA profile by a public laboratory (or outsourced by that public lab to a private one) that adheres to the FBI's Quality Assurance Standards. Assuming the laboratory and analyst that generated the profile are adequately credentialed, the resulting DNA profile may then be entered into the Combined DNA Index System (CODIS). CODIS includes DNA profile databases composed at the local, state, and national levels. At the national level, the National DNA Index System (NDIS) facilitates sharing of DNA profiles among participating law enforcement agencies throughout the United States. At each level, profiles are categorized into forensic (crime scene) profiles, offender profiles, and arrestee profiles. As of March 2011, NDIS contained over 9,535,059 offender profiles. CODIS is primarily evaluated by the number of criminal investigations that CODIS aids. As of March 2011, CODIS had assisted more than 135,500 investigations, suggesting that between 1% and 2% of all samples taken from an offender have assisted a criminal investigation. The categories of individuals from whom law enforcement officials may require DNA samples have expanded in recent years. The federal government and most states authorize compulsory collection of DNA samples from individuals convicted for specified criminal offenses, including all felonies in most jurisdictions and extending to misdemeanors, such as failure to register as a sex offender or crimes for which a sentence greater than six months applies, in some jurisdictions. In addition, the federal government and some states now authorize compulsory collection from people whom the government has arrested or detained but not convicted. As amended, the DNA Analysis Backlog Elimination Act 2000, discussed below, authorizes compulsory collection from individuals in federal custody, including those detained, arrested, or facing charges, and from individuals on release, parole, or probation in the federal criminal justice system. Under the federal law, if an individual refuses to cooperate, relevant officials "may use or authorize the use of such means as are reasonably necessary to detain, restrain, and collect a DNA sample." State laws vary, but nearly all states authorize compulsory DNA collection from people convicted for specified crimes, and a small but growing number of states also authorize compulsory collection from arrestees. At the federal level, statutory authority for compulsory DNA collection has expanded relatively rapidly. During the 1990s, a trio of federal laws created the logistical framework for DNA collection, storage, and analysis. The DNA Identification Act of 1994 provided funding to law enforcement agencies for DNA collection and created the FBI's Combined DNA Index System to facilitate the sharing of DNA information among law enforcement agencies. Next, the Antiterrorism and Effective Death Penalty Act of 1996 authorized grants to states for developing and upgrading DNA collection procedures, and the Crime Identification Technology Act of 1998 authorized additional funding for DNA analysis programs. The resulting framework centers on CODIS; more than 180 law enforcement agencies throughout the country participate in the system. In recent years, federal and state laws have expanded law enforcement authority for collecting DNA in at least two ways. First, laws have increased the range of offenses which trigger authority for collecting and analyzing DNA. In the federal context, the DNA Analysis Backlog Elimination Act of 2000 limited compulsory extraction of DNA to people who had been convicted of a "qualifying federal offense." Under the original act, "qualifying federal offenses" included limited but selected felonies, such as murder, kidnapping, and sexual exploitation. After September 11, 2001, the USA PATRIOT Act expanded the "qualifying federal offense" definition to include terrorism-related crimes. In 2004, the Justice for All Act further extended the definition to reach all crimes of violence, all sexual abuse crimes, and all felonies. Similarly, almost all states now authorize collection of DNA from people convicted of any felony. Second, laws have authorized compulsory DNA collection from people who have been detained or arrested but not convicted on criminal charges. The DNA Fingerprinting Act of 2005 authorized collection "from individuals who are arrested or from non-U.S. persons who are detained under the authority of the United States." The Adam Walsh Child Protection and Safety Act of 2006 subsequently substituted "arrested, facing charges, or convicted" for the word "arrested" in that authority. The U.S. Department of Justice implementing regulations took effect January 9, 2009. Mirroring the statutory language, it requires U.S. agencies to collect DNA samples from "individuals who are arrested, facing charges, or convicted, and from non-United States persons who are detained under authority of the United States." As mentioned, some states have likewise enacted laws authorizing collection of arrestees' DNA. Legislation was proposed in both the 111 th and 112 th Congress to provide incentives to encourage states to establish processes for collecting DNA from persons arrested for specified state offenses. As discussed below, courts measure the intrusiveness of DNA databanking programs by considering both the circumstances under which a DNA profile was collected and the uses to which that DNA profile can be put upon its inclusion in a database. It is, therefore, noteworthy that, in addition to expanding the number and variety of circumstances under which DNA profiling is statutorily required, policymakers have expanded the number and variety of purposes for which databanked profiles can be used.