Highways, railroads, pipelines—we see or hear about these types of physical networks almost every day. But information systems, such as the Internet, and payment systems, such as ATMs and credit cards, also involve networks. Hence, understanding the economics of networks and the unique features of network-dependent industries is crucial to modern life. In this article, James McAndrews outlines some of the unique features of network-dependent industries. He also analyzes some related payment-system issues and demonstrates that determining appropriate public policy would be difficult without a knowledge of the economics of payment networks.