Imagine you are given a long video, possibly thousands of hours in duration, depicting every day scenes like those shown in figure 1. You are asked to analyze the video to find unusual events. In surveillance applications unusual events are those that should be reported for further examinations. What makes unusual events hard to detect? Unusual events are rare, difficult to describe, hard to predict and can be subtle. However, given a large number of observations it is relatively easy to verify if they are indeed unusual.