Everyone now knows that radar has not fulfilled expectations as a means of preventing collision at sea, and that, because of this, its legal status is merely that of an aid to navigation. Nevertheless, everyone seems to feel that such a good technical instrument should enable ships to proceed safely in fog without reducing speed, and the fact that the collisions per fog day have increased by about thirty times in the fifteen years since radar became operational, shows that many mariners are in fact relying on it for collision avoidance. During these fifteen years, many papers have been published pointing out the dangers and giving advice, but in spite of this, the number of vessels involved in collision every year seems to have settled down to a constant figure of about 1700, or 7 per cent of the world fleet.