Project Summary On a moment-by-moment basis each animal must consider its environment, past experience, and present goals to choose future actions. For example, presented with food a hungry animal will likely eat, although not if a predator is near or if the context in which the food appears was previously associated with an adverse output. The process of action selection depends on the proper function of the basal ganglia (BG). Perturbations of BG contribute to many diseases including Parkinson's, Huntington's, Tourette's, and obsessive-compulsive disorders as well as to drug addiction. How activity in the BG mediates action selection and reinforces the repetition of actions associated with positive outcomes is still unknown. Work from a variety of species, indicates that the entopeduncular (EP) and parafascicular (PF) nuclei are important for these processes and contribution to action outcome evaluation and action selection, respectively. Here we propose to carry out anatomical, functional, and behavioral studies in mice to understand the circuitry of these nuclei and how their activity contributes to action selection and evaluation. We make use of new tools and behavioral paradigms to address these unknowns and use anatomical studies to guide our in vivo analyses.