The present study examined the hypotheses that more secure representations of attachments to parents are associated with less adolescent substance use over time and that this link is mediated through relationship quality and monitoring. A sample of 200 adolescents (M = 14–16 years), their mothers, and close friends were assessed over 2 years. Higher levels of security in attachment styles, but not states of mind, were predictive of higher levels of monitoring and support and lower levels of negative interactions. Higher levels of security in attachment styles had an indirect effect on changes in substance use over time, mediated by maternal monitoring. These findings highlight the roles of representations of attachments, mother–adolescent relationship qualities, and monitoring in the development of adolescent substance use.