Project Summary Alcohol use disorder has a major impact on public health, yet the brain mechanisms driving alcohol misuse are poorly understood. Habits are repetitive motor sequences that persist despite reward devaluation and form the action strategy that underlies compulsive behavior. Chronic alcohol exposure facilitates habit learning and leads to increased habitual behavior in chronic users. The dorsolateral striatum (the putamen in humans) regulates habit learning and its resident inhibitory cells, the fast-spiking interneurons (FSIs), are targeted by alcohol exposure. To determine if alcohol utilizes FSIs to promote compulsive alcohol consumption, I selectively ablated striatal FSIs in animals undergoing a voluntary intermittent drinking paradigm (Drinking in the Dark) and challenged animals with the adulterant quinine to measure compulsive consumption. FSI ablation abolished compulsive alcohol consumption and significantly disrupted organized ethanol lick sequence behavior. The next essential step toward advancing this finding to a clinical application is to determine the specific time window that FSIs are recruited for the formation or maintenance of the motor sequences that underlie compulsive drinking. To this end I propose two aims of investigation using innovative optogenetic and machine learning approaches: 1) to determine if striatal FSIs are necessary for the development of the organized actions of compulsive ethanol consumption and; 2) to determine if striatal FSIs are necessary for the maintenance of organized actions underlying compulsive ethanol consumption. The results of this study will significantly advance our understanding of motor sequence learning, the role of motor sequences in compulsive behavior, and will indicate the necessary time window for future therapeutic interventions targeting compulsive drinking.