The current study expands on previous research by using quasi-experimental methods that leverage the experimental context of the Head Start Impact Study (HSIS; Puma et al., 2010) to understand the extent to which Head Start classroom quality moderates the impact of weekly hours in Head Start on children's early language and math skills and externalizing behaviors. We begin by replicating Li and colleagues' (2013) instrumental variables (IV) analysis assessing the effects of weekly hours in Head Start on outcomes for children, leveraging the random assignment nature of the HSIS design and the "offer" of differing numbers of hours of Head Start in the treatment condition and zero hours of Head Start in the control condition. We then extend this work to account for the quality of the Head Start center children attend. We hypothesize that weekly hours in Head Start will be more strongly associated with outcomes for children enrolled in high quality programs as compared with children enrolled in low quality programs. In contrast to previous research that used samples of children enrolled in child care, the current study relies on a sample of children enrolled in educationally focused Head Start programs. (author abstract)