How do you examine six million cargo containers a year? This is the problem confronting U.S. Homeland Defense planners. Each year, over 50,000 ships enter an American port and drop off  these six million containers. Although there are 361 seaports in the U.S., 25 of them account for 98 percent of the container traffic. Taking advantage of this concentration of activity, intelligence collecting activities are being concentrated on these ports, and to the overseas ports from which the containers are shipped. Using profiles of known terrorists, and the composition of the work force in places that load containers headed for the U.S., it is possible to narrow down the number of likely sources of containers with weapons or other materials terrorists might use. This makes it possible to get the most out of limited information collecting resources. Keeping an eye on containers isn't enough, however, because there's also the possibility of chemical, biological or nuclear weapons being put in a container and then detonated before the ship can dock and have its containers examined. This is another reason why the intelligence effort is moving overseas to the shipping ports, not the receiving ones in the United States.