Every day, more than a hundred tank ships and thousands of other vessels make their way into and out of the United States' ports and harbors. The cargo and passengers carried by these ships are important to the U.S. economy. Half of the nation's petroleum consumption and more than 90 percent (by weight) of its foreign trade are carried in ships. Accidents of various kinds happen to these ships with some frequency. We are interested here in navigational accidents-groundings, collisions, and rammings-which constitute a large portion of commercial maritime casualties in U.S. waters and result in losses of several billion dollars each year. In particular, we focus on the expected safety benefit and cost of new technologies, such as electronic charts and integrated navigation systems, which can help reduce these losses by preventing accidents.