Electronic surveillance is increasingly fertile area of litigation as modern technology poses new problems and offers fresh investigatory tools for police and security forces around the globe. The most common method of electronic surveillance is wiretapping in which police or other state agents record telephone conversations of third parties.
Specific Country Examples
The authorities can receive permission to conduct a wiretap under the Indian Telegraph Act of 1885 Part 2 Section 5(2):
On the occurrence of any public emergency, or in the interest of the public safety, the Central Government or a State Government or any officer specially authorized in this behalf by the Central Government or a State Government may, if satisfied that it is necessary or expedient so to do in the interests of the sovereignty and integrity of India, the security of the State, friendly relations with foreign States or public order or for preventing incitement to the commission of an offence, for reasons to be recorded in writing, by order, direct that any message or class of messages to or from any person or class of persons, or relating to any particular subject, brought for transmission by or transmitted or received by any telegraph, shall not be transmitted, or shall be intercepted or detained, or shall be disclosed to the Government making the order or an officer thereof mentioned in the order: Provided that press messages intended to be published in India of correspondents accredited to the Central Government or a State Government shall not be intercepted or detained, unless their transmission has been prohibited under this sub-section.
Evidence obtained through a wiretap cannot be presented as primary evidence at trial.
Despite the Constitutional issues surrounding wiretapping, much of wiretapping law is regulated by the Federal Government under Title III of the Omnibus Crime Control Act and Safe Streets Act of 1968.
The United State Supreme Court in Katz v. United concluded that the Fourth Amendment of the United States Constitution proteted telephone calls made within phone booths. The decision rested on two principles:
First, the Court concluded that "[t]he Government's activities in electronically listening to and recording the petitioner's words violated the privacy upon which [the defendant] justifiably relied while using the telephone booth and thus constituted a 'search and seizure' within the meaning of the Fourth Amendment."
The Court introduced a new model for determining whether a right to privacy existed. In prior cases, the Fourth Amendment was thought to protect activities that occurred only in certain locations, such as private homes or automobiles. The Court determined that this location-based scheme was not always justified, concluding that the appropriate test was not the location. but whether the conversation was made with a "reasonable expectation of privacy." This expectation has two components. Under the subjective component the defendant must wish to make their conversation private. This desire for privacy must also be a reasonable one that society is willing to support. Under this new holding the Fourth Amendment was said to protect people, not places.
The Court further restricted the use of electronic surveillance in U.S. v. Karo, 468 U.S. 705 (1984). In Karo, the Court addressed whether an electronic beeper, installed in a container of chemicals with the consent of the owner, violated the defendant’s 4th amendment rights and whether the information provided by the beeper falls under the protection of the 4th amendment when it provides information that could not have been gathered through visual surveillance. The Court concluded that the installation alone did not violate the defendant’s 4th amendment rights, stating:
The mere transfer to Karo of a can containing an unmonitored beeper infringed no privacy interest. It conveyed no information that Karo wished to keep private, for it conveyed no information at all. To be sure, it created a potential for an invasion of privacy, but we have never held that potential, as opposed to actual, invasions of privacy constitute searches for purposes of the Fourth Amendment. A holding to that effect would mean that a policeman walking down the street carrying a parabolic microphone capable of picking up conversations in nearby homes would be engaging in a search even if the microphone were not turned on. It is the exploitation of technological advances that implicates the Fourth Amendment, not their mere existence. 
The information provided by the beeper, however, was protected by the 4th amendment.
In this case, had a DEA agent thought it useful to enter the Taos residence to verify that the ether was actually in the house and had he done so surreptitiously and without a warrant, there is little doubt that he would have engaged in an unreasonable search within the meaning of the Fourth Amendment. For purposes of the Amendment, the result is the same where, without a warrant, the Government surreptitiously employs an electronic device to obtain information that it could not have obtained by observation from outside the curtilage of the house. The beeper tells the agent that a particular article is actually located at a particular time in the private residence and is in the possession of the person or persons whose residence is being watched. Even if visual surveillance has revealed that the article to which the beeper is attached has entered the house, the later monitoring not only verifies the officers' observations but also establishes that the article remains on the premises. Here, for example, the beeper was monitored for a significant period after the arrival of the ether in Taos and before the application for a warrant to search. 
More recently, in Kyllo v. U.S. 533 U.S. 27 (2001), the Court again addressed electronic surveillance. The question presented in Kyllo was whether the government could use an electronic device, thermal imaging, that was not available to the general public to gather information on a private home that would had previously required physical intrusion. Finding that the use of such devises constitutes a search under the 4th amendment, and thus, presumptively unreasonable without a warrant, the Court wrote:
While it may be difficult to refine Katz when the search of areas such as telephone booths, automobiles, or even the curtilage and uncovered portions of residences is at issue, in the case of the search of the interior of homes-the prototypical and hence most commonly litigated area of protected privacy-there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical “intrusion into a constitutionally protected area,” Silverman, 365 U.S., at 512, 81 S.Ct. 679, constitutes a search-at least where (as here) the technology in question is not in general public use. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. On the basis of this criterion, the information obtained by the thermal imager in this case was the product of a search. 
- Katz v. United States, 389 U.S. 347 (1967)
- U.S. v. Karo, 468 U.S. 705, 712 (1984)
- U.S. v. Karo, 468 U.S. 705, 715 (1984)
- Kyllo v. U.S., 533 U.S. 27, 34-35 (2001)