Surveillance is the monitoring of behavior. Systems surveillance is the process of monitoring the behavior of people, objects or processes within systems for conformity to expected or desired norms in trusted systems for security or social control. Clinical surveillance refers to the monitoring of diseases or public health–related indicators (for example symptoms indicating an act of bioterrorism) by epidemiologists and public health professionals. Although the word surveillance in French literally means "watching over", the term is often used for all forms of observation or monitoring, not just visual observation. Nevertheless, the all-seeing "eye in the sky" is still a general icon of surveillance. Surveillance in many modern cities and buildings often uses closed-circuit television cameras. Although surveillance can be a useful tool for law enforcement and security companies, many people have concerns about the loss of privacy. Surveillance equipment should always be used within the law of the country.

The word surveillance is commonly used to describe observation from a distance by means of electronic equipment or other technological means. For example:

telephone tapping
directional microphones
covert listening devices or "bugs"
Minox subminiature cameras
closed-circuit television
GPS tracking
Bait car
electronic tagging
CCTV Images
military reconnaissance
Reconnaissance aircraft, e.g. Lockheed U-2
Reconnaissance satellites
"trusted" computing devices
Internet and computer surveillance
However, surveillance also includes simple, relatively no- or low-technology methods such as direct observation, observation with binoculars, postal interception, or similar methods.