Motion detection includes methods by which motion has be identified. Motion detection sensors are available which can easily detect sound and produce an alarm or switch on an image recording device. There are motion detectors which employ cameras connected to a computer and this detects certain objects moving.

A simple algorithm for motion detection by a fixed camera compares the current image with a reference image and simply counts the number of different pixels. Since images will naturally differ due to factors such as varying lighting, camera flicker, and CCD dark currents, pre-processing is useful to reduce the number of false positive alarms.

More complex algorithms are necessary to detect motion when the camera itself is moving, or when the motion of a specific object must be detected in a field containing other movement which can be ignored. An example might be a painting in an art gallery surrounded by visitors.