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Surveillance is the act of observing another in order to gather evidence. Surveillance is one of the most common methods law enforcement officials use to investigate suspects and gather evidence. It can be accomplished with the knowledge of the person being surveilled (overt surveillance) or without (overt surveillance). 

Generally, surveillance is electronic or fixed. Electronic surveillance includes wiretapping, bugging, videotaping, geolocation tracking, data mining, social media mapping, and the monitoring of data and traffic on the internet. Fixed surveillance includes covert surveillance of individuals in-person, also referred to as “stake-outs.” 

Individuals and law enforcement officials cannot conduct surveillance without limits, however. Constitutionally, the Fourth Amendment protects individuals from unreasonable searches and seizures, and this can protect individuals against surveillance. For example, in Kyllo v. U.S. (2001), the U.S. Supreme Court addressed the constitutionality of using technology to survey the inside of a defendant's home without actually entering the home. There, the Court held that physical invasion was not required to constitute a Fourth Amendment search if the surveillance reaped information that would not have been attainable without entering the home. Also, some surveillance may require a warrant. For example, electronic surveillance is considered a search under the Fourth Amendment, so it is subject to the same warrant requirements as other searches. To obtain a warrant, the government must show probable cause to believe a search is justified, describe in particularity the activity to be surveilled, and provide a specific time period for the surveillance, among other requirements. In addition to constitutional limitations, Congress also passed the Electronic Communications Privacy Act (ECPA), which regulates electronic surveillance and provides a cause of action to individuals victimized by unlawful electronic surveillance. 

[Last updated in October of 2021 by the Wex Definitions Team]