1. A method for use in a video surveillance system, the method comprising generating at least one hypothesis comprising a set of hypothesized trajectories of objects identified, based on their appearance, as being in a particular object class and hypothesized to have been moving through an area under surveillance at a particular time, said area under surveillance being divided into at least two zones, utilizing said hypothesis to determine whether at least one predefined alert condition has occurred within the area under surveillance, said at least one alert condition including a predetermined pattern of movement of at least one of said objects between at least two of said zones.