This journal article describes a study that applied social network theory to understand how different types of social network norms, along with individual demographic and socioeconomic variables, are associated with recent use of methamphetamine, heroin, and intravenous drugs among homeless youth. The study assessed the relative influence of both injunctive and descriptive norms in the context of different referent groups (i.e., family, street peers, home-based peers, and staff members) on methamphetamine, heroin, and injection drug use behaviors of homeless youth in the past 30 days. The researchers collected cross-sectional data from a sample of 911 homeless youth, ages 14 to 25, at three drop-in centers in Los Angeles. The study consisted of two parts: a social network interview and a computerized self-administered survey. Findings indicate the need to carefully consider the diversity of homeless youths’ networks in designing substance use interventions. Furthermore, the researchers contend that the injunctive norms (i.e., objections to drug use) are associated with reduced drug use and should be considered when developing substance use interventions. Most interventions use social or descriptive norms that focus on changing the perceptions of the prevalence of drug use behavior or altering the drug user’s existing social network.