Surveillance is the monitoring of behavior, activities, or information for the purpose of information gathering, influencing, managing or directing. This can include observation from a distance by means of electronic equipment, such as closed-circuit television (CCTV), or interception of electronically transmitted information, such as Internet traffic. It can also include simple technical methods, such as human intelligence gathering and postal interception.

Surveillance is used by governments for intelligence gathering, prevention of crime, the protection of a process, person, group or object, or the investigation of crime. It is also used by criminal organizations to plan and commit crimes, and by businesses to gather intelligence on their competitors, suppliers or customers. Religious organisations charged with detecting heresy and heterodoxy may also carry out surveillance. Auditors carry out a form of surveillance.

Surveillance can be used by governments to unjustifiably violate people's privacy and is often criticized by civil liberties activists. Liberal democracies may have laws that seek to restrict governmental and private use of surveillance, whereas authoritarian governments seldom have any domestic restrictions. International espionage seems to be common among all types of countries.