This paper presents a method for defining one or more virtual restricted zones within a surveillance area which is observed with stereo cameras. When an object enters a restricted zone, the system highlights the object shown in the monitoring screen or triggers other devices to produce a visual or auditory alarm. The proposed method works by extracting the foreground objects for both the left and the right images from their respective stereo cameras. Then it estimates the object’s position in terms of depth plane using image shifting and number of overlapping pixels. Finally, it determines whether there is a collision between objects and restricted zones in order to trigger an alarm where necessary. The algorithm has been tested with a series of stereo videos, in which samples of it are presented in this paper.