As we near the end of the century, we find ourselves increasingly surrounded by and dependent upon networks. Not only are our computers connected by data networks, our cities by highway networks, and our homes by telephone networks, but myriad invisible networks affect other aspects of our lives. Consider, for example, the economic networks that describe the flow of goods and services or the migration networks that represent the flow of people as they move from place to place. These are but a few of the networks involved in modern life.