The authors tested a mediational model of temperament dimensions and substance use with a sample of 1,826 urban adolescents, M age 12.3 years. Five scales from the Revised Dimensions of Temperament Survey (DOTS-R) were administered together with measures of substance (tobacco, alcohol, marijuana) use and measures of related variables derived from a self-regulation model. Unique contributions to substance use were found for DOTS-R dimensions of high activity level (positively related) and positive mood (inversely related). High activity level and low positive mood were also related to lower levels of parental support. Analyses, including multiple regression and structural modeling, identified generalized self-control, maladaptive coping (anger and helplessness), novelty seeking, and affiliation with peer substance users as mediating the effect of temperament on substance use, with control for effects of parental support. Parental support was inversely related to substance use through several pathways. Implications for the theory of vulnerability are discussed.