Over the past decade there has been renewed research interest in the relative impact of peer selection and peer influence on similarity in the substance use of teenagers. Researchers have concluded that either peer selection or peer influence explain the similarity. Teenagers and Substance Use examines this research question differently, using complete network data, and arrives at different conclusions. Similarity in the substance use of teenagers and their peers is explained by either peer selection or peer influence for some, by both for others, and by neither for others. Using a sociological approach and a social network perspective this book demonstrates how chain reactions result in similarity in the substance use of teenagers, by drawing together those who are similar with those who are not. Chain reactions involve peer selection, the patterning of peer ties and peer influence, and gender affects all aspects of the chain reaction process. Although their peers are the main influencers, family, relatives and other people also affect the substance use of teenagers.