Adolescent siblings are often similar in a variety of adjustment outcomes, yet little is known about the processes that explain sibling influences during adolescence. Two alternative explanations were tested, attachment (based in social bonding theory) and anaclitic identification (based in social learning theory). Hypotheses were tested with a sample of 613 adolescent sibling pairs (206 nonadopted, 407 adopted; elder sibling mean age = 16.1 years, younger sibling mean age = 13.8 years) across three sibling contexts (gender composition, age difference, and genetic similarity). Attachment explanations were supported so that the greater the perceived sibling emotional and behavioral closeness, the lower the likelihood of substance use; however, there were considerable moderating effects of sibling gender composition. Anaclitic identification explanations were not supported; closeness and elder sibling substance use did not interact to predict younger sibling substance use. Overall, this research adds to a body of work demonstrating important sibling influences on adolescent substance use.