Specifically, this dissertation first develops a heuristic theoretical model that describes the pathways from policy intervention, through high quality ECE, to young children's learning and development and then tests components of this model using data from the national Head Start Impact Study (HSIS; Puma et al., 2010). To test elements of this theory of change, the empirical papers in this set of work examine 1) mediating pathways within ECE programs through which external policies may be associated with classroom quality and 2) state policies' associations with children's access to formal and high quality ECE. This research helps to fill a critical knowledge gap and expand the field's understanding of which policy levers may be most reliably related to improvements in quality within ECE programs. (author abstract)