The first aim of this revised proposal is to accurately assess the duration and trajectories of smoking abstinence symptoms and associated biobehavioral indices across 66 days of quitting smoking in 3 different treatment groups: 1) bupropion SR (BUP), 2) transdermal nicotine patch (TNP), and 3) placebo patch plus placebo pill. Study sensitivity and accuracy will be maximized by using procedures designed to maximize abstinence and minimize study dropout. The second aim is to characterize brain and psychological mechanisms by which BUP and TNP promote abstinence. While the efficacies of BUP and TNP in promoting smoking abstinence have been repeatedly demonstrated, little is known about the mechanisms mediating this efficacy. The final primary goal of this competitive continuation proposal is to characterize individual differences in psychological and brain mechanisms mediating the beneficial effects of BUP and TNP on smoking abstinence and withdrawal symptoms. A secondary goal is to assess the ability of a battery of innovative brain and biobehavioral measures of attention and affect to predict relapse.