In adolescence, several factors are salient in predisposing the individual to different types of adjustment problems, including problematic substance use (PSU). It is important to gain insight into the processes that are involved in adolescent PSU, because it can provide entry points for prevention and intervention programs. Therefore, the present PhD thesis focused on the role of temperament and rumination in adolescent substance use and problems, by addressing seven research aims. Temperament is defined as individual differences in affective/motivational aspects of reactivity and self-regulation. The first aim of the PhD thesis was to examine differential relations of affective and motivational aspects of reactivity with PSU, in a community sample as well as in a clinical sample. With regard to the affective aspects of reactivity, high levels of NA predicted more alcohol-related consequences in a community sample of drinking adolescents. Adolescents with substance abuse and/or dependence reported lower levels of PA and higher levels of sad NA compared to their matched controls. In addition, low PA was the most important dimension of affective reactivity in predicting clinical status as a substance user. With regard to the motivational aspects of reactivity, BAS Fun Seeking was found to predict several aspects of alcohol consumption in community samples. Levels of motivational reactivity were not different for clinical substance users compared to their controls. These results suggest that motivational reactivity (and especially BAS Fun Seeking) is predictive of consumption of alcohol, whereas affective aspects are related to more problematic patterns of use. With regard to motivational reactivity, the second aim was to clarify the role of the different aspects of BAS reactivity in alcohol use. Results demonstrate that especially BAS Fun Seeking was related to alcohol use, suggesting that it may be useful to focus on this scale when modeling risk for alcohol use by means of the BIS/BAS Scales. The third aim was to examine if the relations between reactivity and PSU were moderated by EC. Higher levels of BAS Fun Seeking were found to be associated with a higher frequency and quantity of alcohol use at low levels of EC and not at high levels of EC. The fourth aim was to investigate whether the associations between affective/motivational aspects of reactivity and drinking behavior were mediated by drinking motives. Results indicated that the relation between BAS Fun Seeking and alcohol consumption was mediated by enhancement (i.e., drinking to enhance positive affective states) and social motives (i.e., drinking to obtain positive social rewards), whereas the association between NA and alcohol-related consequences was mediated by coping-depression motives (i.e., drinking to diminish negative affective states). According to Nolen-Hoeksema (1991) Rumination is defined as the tendency to repetitively and passively focus on symptoms of distress and on the possible causes, and consequences of these symptoms. Rumination has been most extensively studied in relation to emotional problems. However, there is also evidence that rumination may increase the risk for PSU. The fifth aim of the present PhD thesis was to investigate the cross-sectional associations between subtypes of rumination (i.e., brooding and reflection) and PSU. Results indicated that high levels of brooding were related to more substance use problems and lower levels of reflection were related to a higher drug consumption. All relations remained significant when controlling for depressive symptoms, indicating the unique associations between rumination subtypes and (problematic) substance use. Given the importance of investigating how rumination subtypes and PSU are related across time, the sixth aim was to examine the cross-temporal associations between rumination subtypes and PSU, using cross-lagged path analyses. Results indicated that substance use problems predicted increases in levels of brooding and reflection over time. Given the well documented gender differences in both rumination and substance use, the seventh aim of the PhD thesis was to investigate whether the associations between rumination subtypes and PSU were moderated by gender. In the cross-sectional study, lower reflection was related to substance use problems, only in boys; whereas in the longitudinal study substance use problems predicted increases in levels of brooding and reflection over time only in girls.