Data collected from 294 young adults, ages 19 to 25, and both a same- and an opposite-gender best friend or mate across 3 annual assessments were analyzed to examine the similarity to and influence of the peer on the young adult's substance use. The authors found similarity across time between both peers and the young adult in cigarette use, alcohol use, binge drinking, and, in most cases, marijuana use. In prospective analyses, peer use predicted young adult cigarette use, binge drinking, and problem use by the young adults. Results were generally consistent across gender and for both same- and opposite-gender peers. Findings emphasize peer influence contribution to young adult substance use and suggest the design of interventions that involve both young adults and their peers.