The question is being asked more and more, why, despite the use of radar, collisions at sea in fog still occur. There are evidently many answers to this question and all or nearly all of them hinge on some aspect or other of human fallibility. Yet these answers may be grouped into two distinct categories, one of which covers failure to get the most out of the radar and ancillary equipment, and the other the problems of behaviour with which the radar information brings the mariner face to face in fog for the first time.