The association between alcohol use and smoking was examined in a large population-based sample of Dutch twins consisting of three age groups; young adolescent twins aged 12-14 years (n = 650 twin pairs), 15-16-years-old adolescent twins (n = 705 twin pairs), and young adult twins aged 17-25 years (n = 1266 twin pairs). For all three age groups, alcohol use and smoking were correlated (r = 0.5-0.6). Adolescents and young adults who smoked were more likely to drink alcohol than nonsmokers. The relation between alcohol use and smoking was also found within a twin pair; alcohol use in one twin was correlated with smoking in the cotwin. This finding suggested that familial factors contribute to the association between alcohol and tobacco use. With a bivariate genetic model, it was examined to what extent the comorbidity was due to genetic and environmental factors that predispose to both alcohol use and smoking. The genetic analyses showed that the underlying factors that influence alcohol and tobacco use and cause their association were different for adolescent and young adult twins. Initiation of alcohol use and smoking in adolescents (aged 12-16 years) was substantially influenced by the same shared environmental features. Alcohol and tobacco use in young adults were associated due to the same genetic risk factors.