Surveillance is the systematic observation and monitoring of a person, population, or location, with the purpose of information-gathering, influencing, managing, or directing.

It is widely used by governments for a variety of reasons, such as law enforcement, national security, and information awareness. It can also be used as a tactic by persons who are not working on behalf of a government, such as by criminal organizations to plan and commit crimes, and by businesses to gather intelligence on criminals, their competitors, suppliers or customers. Religious organizations charged with detecting heresy and heterodoxy may also carry out surveillance. Various kinds of auditors carry out a form of surveillance.

Surveillance is done in a variety of methods, such as human interaction, postal interception, closed-circuit television (CCTV) cameras, wiretapping, and data mining.

Surveillance can unjustifiably violate people's privacy and is often criticized by civil liberties activists. Democracies may have laws that seek to restrict governmental and private use of surveillance, whereas authoritarian governments seldom have any domestic restrictions. Increasingly, government and intelligence agencies have conducted surveillance by obtaining consumer data through the purchase of online information. Improvements in the technology available to governments have led to mass surveillance and global surveillance.

Espionage is by definition covert and typically illegal according to the rules of the observed party, whereas state authorities typically regulate surveillance and consider many uses legal or legitimate.