Research has found that children who attend Head Start for more than one year tend to score higher on cognitive skills at the end of pre-K or at kindergarten entry than children who attend for one year or less (Lee 2011; Wen et al. 2012b; Xue et al. 2016). Although attending Head Start for more years is linked with higher cognitive scores, children's gains across the years may not be comparable (Yoshikawa et al. 2013). In nationally representative data from the Head Start Family and Child Experiences Survey 2014-2018 (FACES 2014), the authors found that, on average, first-year children showed larger gains in language and cognitive scores from fall to spring of the program year than second-year children (Kopack Klein et al. 2018). In this brief, the authors use data across one year of Head Start (from fall 2014 to spring 2015) from FACES 2014 to expand this finding and explore possible explanations for why second-year children made smaller gains in language and cognitive scores than first-year children during the Head Start program year (see methods in the box at the end of the brief). They explore whether differences between first- and second-year children's gains across the program year can be explained by differences in child and family characteristics, including age; fall language and cognitive scores; part-day enrollment; classroom quality; or teacher experience and education.