The 1998 reauthorization of Head Start called for a national evaluation of the Head Start program. The goal of Head Start is to improve the school readiness skills of low-income children. Yet characteristics of Head Start programs, such as their quality and the amount of time children spend in them may influence their effectiveness at achieving this goal. Previous research has demonstrated that children who spend more weekly hours in Head Start demonstrate larger cognitive gains than children who attend fewer weekly hours (Li, Farkas, Duncan, Vandell, & Burchinal, 2013). But the quality of care children attend during such hours may also matter, as there is mounting evidence regarding the importance of classroom quality for children's developing school readiness skills (Burchinal, Vandergrift, Pianta, & Mashburn, 2009; Zaslow et al., 2010). Thus, the effect of weekly hours in Head Start may vary by the quality of the Head Start classroom. Indeed, previous research has found that child care quality moderates the association between hours in child care and child behavior problems, but not cognitive skills (McCartney et al., 2010; Votruba-Drzal, Coley, & Chase-Lansdale, 2004). 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. The authors 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 3- and 4-year-old children enrolled in educationally focused Head Start programs in 22 states. Four tables are appended.