AbstractTheInternationalRegulationsfor Prevention of Collisions at Sea (33 U.S.C. 1051), the Inland Rules (33 U.S.C. 154), the Pilot Rules, Great Lakes Rules, Western Rivers Rules and the Admiralty Court decisions which interpret them, form a real and complex system of constraints upon the mariner in the navigable waters of the United States and on the open sea. These Rules are law, both international and U. S., and are used by the Courts in the determination of liability in case of collision. Admiralty Court decisions have modified and clarified the Rules of the Road in many cases. They have also added to the difficulty of the watch officer on the bridge of a ship at sea who must know not only the Rules but the Court interpretations in order to comply with them. The nature of the constraints imposed by these Rules is examined as they apply to the information furnished to the mariner from marine radar and radar based collision avoidance systems. The rules which apply when the vessels are in sight of each other and when they are not are discussed. The information needed to comply with the rules in each case is compared to the information available from radar and collision avoidance systems. The informational short comings of these systems are identified and an ideal collision avoidance system to aid the mariner in maneuvering under the constraint of the Rules of the Road is described.