The purpose of this study is to investigate the neurobiological mechanisms associated with cigarette use in young nondaily smokers (ages 21-25). Nondaily smokers experience fewer symptoms of cigarette withdrawal than daily smokers; however, they still experience difficulty quitting. Since smoking cessation before the age of 30 substantially attenuates the long-term health consequences associated with smoking, understanding the factors that motivate cigarette use in young nondaily smokers and developing tailored interventions will be critical for curbing cigarette use and improving public health. The proposed project will use functional magnetic resonance imaging (fMRI) to examine how other substances (e.g. stimulates, sedatives, or analgesics) impact brain function and will assess the extent to which substance-induced changes in brain function correlate with impulsive choice, cigarette craving, and craving regulation in young adults. Furthermore, the investigators will assess how substance-induced changes in brain function, impulsivity, craving, and capacity for craving regulation predict cigarette use over the course of 1 year.