1. An eye tracking system for monitoring the movement of a user's eye, the system comprising: (a) video data input means for receiving video data produced by eye imaging means monitoring the user's eye; (b) spot location means for determining from the video data the location of a reference spot formed on the user's eye by illumination of the user's eye by a point source of light, the spot location means including adaptive threshold means for providing an indication of parts of the image produced by the eye imaging means which have a brightness greater than a threshold value, and spot identification means for selecting a valid reference spot by comparing said parts of the image with predetermined validity criteria; (c) pupil location means for determining from the video data the location of the centre of the pupil of the user's eye relative to the reference spot in order to determine the user's line of gaze, the pupil location means including selection means for selecting a pupil tracking window comprising a portion of the image produced by the eye imaging means containing the pupil relative to the valid reference spot location, edge determination means for determining an edge of the pupil by selection of those parts of said image portion having a gradient greater than a threshold value, and centre determination means for location of the centre of the pupil by making reference to selected points about the edge of the pupil; and (d) display means for indicating the user's point of regard from the user's line of gaze determined by the pupil and spot location means.