The marriage of the computer with the collision avoidance (C/A) radar permits a higher order of manœuvrability finesse than is possible with the plotting board. Given the computer as a new tool, the marine systems engineer is faced with the question of whether to attempt a radical improvement in the art of collision avoidance displays, or whether merely to introduce a marginal extrapolation of the existing art. That the first course of action may be necessary is evident from various studies of the modern marine traffic environment; thus a recent study by the United States Coast Guard for the Department of Transportation showed that ‘…the total probability of collision avoidance for this condition [surrounded by a significant number of ships] requires an analytical solution quite different from one which would relate to a sparsely populated area’. Indeed the increase in shipping density, collision rates, speed and size of the newer ships, economic pressures and ecological constraints demand a radically different approach to solving the collision avoidance problem. The marine electronics industry has responded by developing collision avoidance systems of various types. Many of these are relatively straightforward extrapolations of existing art but one at least, the Sperry CAS, goes beyond a simple extrapolation and is a radical departure from conventional practice.