Closed Circuit Television (commonly abbreviated as CCTV) is a video surveillance technology designed to monitor a variety of environments and activities. It is called a "closed circuit" since, unlike what happens with diffusion, all its components are linked. Furthermore, unlike conventional television, this is a system designed for a limited number of viewers.[1]​.

The circuit can simply be made up of one or more surveillance cameras connected to one or more video monitors or televisions, which reproduce the images captured by the cameras. Although, to improve the system, other components such as videos or computers are usually connected directly or networked.

They are fixed in a certain place. In a modern system the cameras used can be controlled remotely from a control room, where their pan, focus, tilt and optical magnification can be configured. This type of camera is called PTZ (pan-tilt-zoom). These systems include night vision, computer-assisted operations and motion detection, which makes it easier for the system to put itself on alert when something moves in front of the cameras. The clarity of the images can be excellent, it can be transformed from dark to light levels. All these qualities mean that the use of CCTV has grown extraordinarily in recent years.

Surveillance through the use of CCTV is increasingly common in public spaces for crime prevention. The rationale for its use is that it deters potential criminals, can quickly alert police or security personnel to intervene, and helps people feel safer. However, the use of CCTV has a high cost. For example, it is estimated that in the United Kingdom more than £250 million was spent on CCTV between 1992 and 2002, with this form of surveillance being one of the most funded.

A systematic review of 44 studies conducted in the United Kingdom, United States, Canada, Norway and Sweden determined that the use of CCTV has a modest impact on reducing crime. The results suggest that surveillance is most effective in preventing vehicle thefts in parking lots, and is less effective in the urban centers of cities and towns, in public housing, and on public transportation. Likewise, it was determined that the effectiveness is greater when the camera coverage is broader, and that it has no effect on violent crimes.[2]​.

Originally, video surveillance was carried out using closed circuit television. This technology uses analog video cameras, coaxial cable and video recorders. The cameras transmit a signal to a specific, limited set of monitors. CCTV systems typically include a fixed communications link between cameras and monitors, using wires and cables.[1]​ This method records what is happening, but does not send you alerts. CCTV works like a video player, you have to change cassettes, picture quality is average, storage tapes wear out over time, and storage is bulky. Additionally, cameras, monitors, and recorders must be physically close enough to be connected by cables.[3]​.