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.

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

• eavesdropping
• 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.