PROJECT SUMMARY/ABSTRACT Withdrawal is a key, multi-faceted component of tobacco/nicotine dependence. Because withdrawal symptoms are theorized to drive relapse, facets of withdrawal (e.g., craving, negative affect) are the targets of most current and emerging treatments. Despite the central importance of withdrawal, and a voluminous literature on withdrawal from combustible cigarette smoking, we know little about withdrawal from electronic nicotine delivery systems (ENDS). To overcome critical barriers to progress in the field, we propose the first-ever prospective, controlled comparison of abstinence- induced withdrawal between ENDS vapers and cigarette smokers. Participants will be 160 established daily vapers (including former smokers and dual users who smoke occasionally), 160 established daily smokers (including former vapers and dual users who vape occasionally), and (for exploratory comparisons) 50 established daily dual users, who smoke and vape daily. Participants will complete two 4-hour lab visits; the order of the ad lib use visit and the abstinent visit (which follows 24 hours of abstinence) will be randomized across participants. To advance knowledge of ENDS withdrawal, we will employ state-of-the-science, multi-measure, multi-method assessments of key withdrawal facets: negative affect, craving, difficulty concentrating, restlessness, sleep, and appetite, as well as anhedonia/positive affect and somatic effects. We hypothesize that, for each facet, withdrawal magnitude will be lower among vapers compared to smokers. To inform theory and intervention development, we will also evaluate the behavioral significance of ENDS withdrawal, testing the hypotheses that abstinence will increase the motivation to vape/smoke and this group difference will be accounted for (mediated) by vaper/smoker differences in one or more withdrawal facets. Exploratory analyses will examine whether group differences in withdrawal are accounted for (mediate) by differential nicotine exposure, explore the role of individual differences (e.g., sex, rate of nicotine metabolism, expectancies), and examine differences among sub-groups of vapers. The impact of this much-needed, detailed characterization of withdrawal from ENDS is enhanced by the inclusion of a comparator of great public health significance, cigarette smoking. In addition, by characterizing the specific withdrawal facets that drive motivation to vape/smoke, the proposed work will identify promising intervention targets for subsequent treatment development efforts.