The Fourth Amendment to the U.S. Constitution protects the “right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures.” It states that warrants, supported by probable cause, must be issued before such searches and seizures can take place.
How does it relate to electronic surveillance?
The Supreme Court held in its 1928 ruling in Olmstead v. US that the Fourth Amendment did not apply to wiretaps of telephones that did not involve the physical entry of homes. But as technology changed, so did the court’s views.
In the 1967 case of Katz v. US, the Supreme Court ruled that the Fourth Amendment protects “people, not places” and that a person in a phone booth has a reasonable “expectation of privacy” and his or her conversation should not be monitored without a warrant.
Congress passed a law in the following year requiring law enforcement to get a warrant before placing a domestic wiretap. The law gave the government some leeway to conduct warrantless surveillance in matters of “national security.” However, in the 1972 case of US v. US District Court, the Supreme Court ruled that the Fourth Amendment required judicial input before the executive branch could make surveillance decisions.
After the extent of warrantless surveillance on Americans was revealed during the Congressional hearings of the mid 70s, Congress passed the 1978 Foreign Intelligence Surveillance Act (FISA) to put a judicial check on spying operations within the United States. The Act set up a secret court to hear requests for warrants in national security matters involving spying by foreign powers and international terrorism.
Under FISA, the executive branch could only carry out electronic surveillance within the US after consulting the FISA court. Anyone who violated the law faced a fine of up to $10,000 or five years in prison. Because the evidence seized through electronic surveillance was not intended to be used in criminal hearings but only for intelligence purposes, the standard for issuing a warrant was less than probable cause. As a result, the FISA court modified only two warrant requests out of 13,102 applications during its first 22 years of operation.
Almost immediately after the September 11 attacks, President Bush set aside both FISA and the Fourth Amendment, and authorized the secret warrantless wiretapping by the NSA of the telephone calls of Americans and foreigners within the US. The New York Times revealed the warrantless surveillance on December 16, 2005 after sitting on the story for a year.
How did the Bush Administration justify breaking the FISA law? It claimed that the President had the “inherent authority” as Commander-in-Chief to ignore the FISA statute, and that he was also given the power to do so by the Authorization for Use of Military Force passed by Congress immediately after 9/11. 
The power given the FISA court to serve as a check on the government’s surveillance powers was subsequently significantly watered down by the FISA Amendments Act of 2008, which gave the NSA the power to conduct dragnet surveillance of Americans’ international phone calls and emails with extremely limited judicial oversight.
Fourth Amendment protections have been undermined in many other ways since 9/11. Sections of the USA PATRIOT Act have broadened police wiretap authority, and expanded the use of “sneak and peek” search warrants allowing secret physical searches of homes and offices with a greatly delayed notification requirement. Advances in digital technology, Department of Homeland Security grants that have networked parts of the country with powerful surveillance cameras, and the Transportation Security Administration’s “enhanced pat downs” and full-body scanning devices have rendered the Fourth Amendment notion of an “expectation of privacy” and freedom from unreasonable searches largely obsolete.
Map image courtesy EFF