This 2006 study examined 1) whether maternal use of tobacco and consumption of alcohol when a child is 5 and 14 years of age predict cannabis use in young adults, and 2) whether this association is explained by possible confounding or mediating factors. Data were taken from a prospective birth cohort study of mothers and their children in Brisbane, Australia. This study was based on a cohort of 3,176 young adults who participated at the 21-year follow-up of the study and for whom data were available on maternal smoking and alcohol consumption 5 and 14 years after their birth. After controlling for possible confounders, the authors found that maternal smoking at 14 years was associated with frequent use of cannabis in offspring at 21 years, regardless of maternal smoking at 5 years. Children of mothers who drank more than one glass of alcohol at 5 years and continued at 14 years were more likely to use cannabis in early adulthood. The association between maternal substance use and offspring cannabis use was partially mediated by adolescent externalizing behavior and smoking measured at 14 years. Prevention programs that address maternal and adolescent tobacco use and adolescent externalizing behavior should be considered as strategies to reduce cannabis use by young adults.