Approximately 700,000 preschool-aged children in the U.S. have a diagnosed special need (6%). As of 2009, 48% of these children received all of their special education services within preschool inclusion classrooms alongside their typically developing peers (U.S. Department of Education, 2012). In stark contrast to the more than 80 rigorous studies since 1965 that have demonstrated the efficacy of preschool in improving the outcomes of typically developing children (Yoshikawa, Weiland et al., 2013), little is known about the causal impacts of inclusion preschool on children with special needs. Even less is known about optimal program dosage or the mechanisms by which such programs promote positive child development. With the generous support of the NAEd/Spencer Postdoctoral Fellowship, I will conduct two rigorous studies on the impacts, dosage, and mechanisms of the inclusion preschool model that may help to meet this critical gap in the literature. In Study 1, I use regression discontinuity to estimate the impacts of inclusion preschool on the cognitive school readiness of young children with special needs. Study 1 also examines whether the gains of enrolled children with special needs are on par with those of their typically developing peers, as well as the added value of two vs. one years of inclusion preschool. Data for Study 1 come from a sample of children enrolled in a highly successful prekindergarten program. In Study 2, using data from the National Head Start Impact Study, I will use principal stratification with Bayesian estimation to examine whether impacts on the cognitive and socio-emotional skills of children with special needs are mediated causally by care-setting support for family involvement. Such support is often challenging for inclusion programs, but is thought to be critical in improving the outcomes of this subgroup. Results from the two studies stand to make significant contributions to educational practice and policy, given the dearth of rigorous evidence on the preschool inclusion model and given current policy proposals to dramatically increase access to preschool.