How do criminals communicate with each other? Unlike the rest of us, people planning crimes can't freely advertise their goods and services, nor can they rely on formal institutions to settle disputes and certify quality. They face uniquely intense dilemmas as they grapple with the basic problems of whom to trust, how to make themselves trusted, and how to handle information without being detected by rivals or police. In this book, one of the world's leading scholars of the mafia ranges from ancient Rome to the gangs of modern Japan, from the prisons of Western countries to terrorist and pedop.