It is well established that there is an increase in substance use among college students. In the literature, this increase in use has been attributed to different personality factors, such as sensation seeking. However, what has not received sufficient attention is the possibility that the new peer groups, afforded by the transition to college, introduce unique influence on the relationship between personality and substance use. The purposes of the current study were to explore whether personality predicted substance use across the transition to college whether peer substance use moderated that relationship. The current study examined developmentally the relations among personality, peers, and substance use as students transitioned to college. It built upon previous work by disentangling how the multifaceted trait of impulsivity may interact with the aspects unique to the transition to college, such as dynamic peer groups and substance use behavior in different contexts. This study added to the literature as it was the first to examine negative urgency and its relation to peer influence. The results of the current study aid in understanding the development of substance use among college students and the environmental contexts likely to influence use across time. Participants (N = 229) were assessed longitudinally in order to examine changes in substance use. The participants completed an array of measures that included personality measures (i.e. sensation seeking and negative urgency), a life history calendar of substance use, a measure of problematic alcohol use, and a questionnaire examining the substance use of peers. The current study suggests differences in the way that sensation seeking and negative urgency predict alcohol use and problematic drinking across the transition to college and demonstrated that peers' drinking had a moderating effect on the relationship between personality and drinking during particular times during the transition to college. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]