In 1967, the Supreme Court pronounced in Katz v. United States that "[w]hat a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection." This rule "that a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties" is known as the "third-party doctrine." While its reach in the pre-digital age was relatively limited, the third-party doctrine has provided the government a powerful investigative tool in a society where people share ever-increasing amounts of information with others. Many have debated whether these technological and social changes require the courts to reconsider this doctrine, or, alternatively, whether Congress should step in and create some form of statutory protection for this information. Over the years, the Court has applied the third-party doctrine to two main sets of cases. In one, the Court has held that people do not have a reasonable expectation that a person with whom they are communicating will not later reveal that conversation to the police. In the second, the Court extended this doctrine to hold that people are not entitled to Fourth Amendment safeguards for records given to a third-party or data generated as part of a person's business transactions with a third-party. In two of the most prominent third-party cases, Smith v. Maryland and United States v. Miller , the Court held that government access to telephone calling records and bank records, respectively, were not Fourth Amendment searches for which warrants were required. To be clear, the third-party doctrine does not cover all conceivable information that is transferred through a third party. For instance, the content of a voice or email communication does not fall within its scope. The courts have reasoned that the service provider is merely the conduit or intermediary of those communications and not the recipient; thus, the user does not lose privacy protection in those communications. On the other hand, both non-content and content information that is shared directly with a service provider is covered by the third-party doctrine (e.g., the deposit slips or checks shared with a bank and data kept by the bank relating to transactions with it). Additionally, non-content information derived from private interactions with others is subject to the third-party doctrine. This covers data such as telephone numbers dialed, email addresses of those emailed, or websites visited. The third-party doctrine has been heavily criticized for unnecessarily constricting Americans' privacy rights. But whatever one thinks of the rule that citizens are not entitled to Fourth Amendment protection when they share information with one another, the third-party doctrine is largely entrenched in other areas of Fourth Amendment case law. For example, it is not a Fourth Amendment search for the police to dig through one's trash left on the curb, to track a person's movements on public streets, and even to surveil a person in a fenced-in backyard with an aircraft. In each of these instances, the Court reasoned that because the person exposed his activities to the public gaze he was no longer entitled to an expectation of privacy. In addition to the legal attacks on the third-party doctrine, some have questioned its practical implications in a society which shares almost every facet of its life with various entities. Both Smith and Miller , decided in the mid- to late-1970s, came before the mass digital revolution experienced over the last several decades. Since these decisions, there has been a wave of advancement in data generation, collection, automation, and processing. Whether these new technologies and shifts in social interaction require courts or lawmakers to revise this review is currently under debate. Two major events in the past few years typify this ongoing debate. The first is the conversation prompted by several concurrences in the 2012 GPS tracking case United States v. Jones . In two concurring opinions in that case, five Justices opined that warrantless, pervasive government location monitoring can violate the Fourth Amendment. Commentators have speculated that these five votes could have significant consequences for other similar ubiquitous surveillance techniques. And at least one member of the Court, Justice Sotomayor, believes that the third-party doctrine should be seriously rethought as a whole. The second is the litigation surrounding the National Security Agency's telephone metadata program. Several federal courts, including the Foreign Intelligence Surveillance Court, have applied Smith and the third-party doctrine to uphold this comprehensive data collection program. One district court judge, however, found Smith outdated and the NSA program too invasive for Smith to still control this legal question. With these shifts in technology and legal thinking in mind, this report explores the history and legal foundations of the third-party doctrine. It will first provide background to the Fourth Amendment and describe in what instances government investigations trigger its protections. It will then analyze the Court's third-party doctrine cases and provide doctrinal and practical arguments for and against its application. Next, this report will examine how Congress has responded to the third-party doctrine and whether United States v. Jones and subsequent cases might alter its future application. Lastly, this report will consider any potential future developments in this fast-moving area of law. Before the advent of modern communications, government officials could not simply subpoena an Internet Service Provider (ISP), or Amazon, or Google for information relating to a target of investigation, but had to enter the suspect's home or office, sometimes by force, to retrieve personal information directly themselves. During the 18 th century, British and colonial officials conducted searches and seizures of people's homes with little to no suspicion of wrongdoing pursuant to either a general warrant, which was used mainly in England, or a writ of assistance, which was used in the American colonies. These indiscriminate government intrusions contributed to the people's fear of unrestrained government power and led to the eventual passage of the Fourth Amendment. Take, for instance, the formative English search and seizure case Entick v. Carrington , where the government was investigating John Entick and others for alleged publication of seditious articles. In that case, government officials broke into Entick's home with "force and arms," pried open the locks on his doors, broke open his chests and drawers, and searched his private papers and books for four hours. The officers conducted this search under the guise of a general warrant, a legal order which states with a high level of generality the places and things to be searched and seized. In outlawing these practices, Lord Camden of the English bench observed: [O]ur law holds the property of every man so sacred, that no man can set his foot upon his neighbour's close without his leave; if he does he is a trespasser, though he does no damage at all; if he will tread upon his neighbour's ground, he must justify it by law. These same intrusive practices also faced disfavor in the American colonies. British officials often resorted to writs of assistance, a form of general warrant, which permitted house-to-house searches. These legal orders generally failed to allege any illegal activity and were not signed off on by a judge. In the famous Paxton's Case , leading Boston attorney James Otis attacked these writs as "the worst instrument of arbitrary power, the most destructive of English liberty, and the fundamental principles of the constitution, that was ever found in an English law book." John Adams later commented that these indiscriminate intrusions were "the spark in which originated the American Revolution." To prevent the newly established federal government from committing these incursions into their lives, the American people ratified the Fourth Amendment as part of the Bill of Rights in 1791. It reads: The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath of affirmation, and particularly describing the place to be searched, and the person or things to be seized. Over the years, the federal courts have struggled to reconcile the first clause of the Amendment, which requires that all searches and seizures be reasonable, with the second clause, which requires that all warrants meet certain minimum requirements such as particularly describing the place to be searched and the things to be seized. In any event, the Court must first determine whether the Fourth Amendment's restrictions apply at all. This is done by asking whether the government has conducted a "search," a legal term of art that cannot be resolved by mere dictionary definition, but instead requires application of the Supreme Court's intricate, and at times contradictory, Fourth Amendment case law. Although the Fourth Amendment was ratified in 1791, the Supreme Court's first in-depth interpretation of what constitutes a Fourth Amendment search did not arise until the 1886 case United States v. Boyd . In Boyd , the government obtained a court order for the Boyds to provide an invoice of goods they imported which the government planned to use against them in court. The Boyds produced the invoice, but protested that its production constituted an unreasonable search and seizure under the Fourth Amendment. Looking to Entick and other pre-Revolutionary cases for guidance, the Court found that the production of private papers was so similar to an actual invasion into one's home that it constituted a Fourth Amendment search. Although Boyd instructed that the Fourth Amendment should be "liberally construed," the Court narrowed the scope of what constitutes a search in Olmstead v. United States . In that case, federal agents investigating the bootlegging activities of a criminal syndicate placed a wiretap on several phone lines running from the homes and office of four suspects. At no point did the officers trespass upon the defendants' property to conduct the tap. The Court held that these wiretaps should not be considered a search as the "Amendment itself shows that the search is to be of material things—the person, house, papers, and effects," and the intangible voice of the defendants was not covered by its literal terms.