‘Six degrees of separation’ is a cliche, as is ‘it’s a small world’, both cliches of the language and cliches of everyone’s experience. We all live in tightly bonded social networks, yet linked to vast numbers of people more closely than we sometimes think. Only in recent years have scientists begun to apply insights from the theoretical study of networks to understand forms as superficially different as social networks and electrical networks, computer networks and economic networks, and to show how common principles underlie them all. Duncan Watts explores the science of networks and its implications, ranging from the Dutch tulip mania of the 17th century to the success of Harry Potter, from the impact of September 11 on Manhattan to the brain of the sea-slug, from the processes that lead to stock market crashes to the structure of the world wide web.