Surveillance is the monitoring of behavior, many 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 like Internet traffic. It can also include simple technical methods, such as human intelligence gathering and postal interception.

Surveillance is used by citizens for protecting their neighborhoods. And by governments for intelligence gathering - including espionage, 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 criminals, 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.

A byproduct of surveillance is that it can 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.

Espionage is by definition covert and typically illegal according to the rules of the observed party, whereas most types of surveillance are overt and are considered legitimate. International espionage seems to be common among all types of countries.