High relapse rates among substance dependent individuals are likely due to a combination of factors that involve limbic circuits in the brain involved in craving, including vulnerability to salient cues. Emerging data suggests that non-invasive, targeted brain stimulation may be able to modulate activity in these circuits and decrease craving. The primary goal of this pilot study is to determine the extent to which a single session of continuous theta burst stimulation (TBS) to the medial prefrontal cortex can attenuate limbic circuitry involved in craving among cocaine users and alcohol users. This will be tested through a double-blind,sham-controlled brain stimulation and brain imaging study in a cohort of polysubstance abusers and alcohol users.