This study examined the effect of school-based kindergarten transition policies and practices on child outcomes. The authors followed 17,212 children from 992 schools in the Early Childhood Longitudinal Study-Kindergarten sample (ECLS-K) across the kindergarten school year. Hierarchical linear modeling revealed that the number of school-based transition practices in the fall of kindergarten was associated with more positive academic achievement scores at the end of kindergarten, even controlling for family socioeconomic status (SES) and other demographic factors. This effect was stronger for low- and middle-SES children than high-SES children. For low-SES children, 7 transition practices were associated with a 0.21 standard deviation increase in predicted achievement scores beyond 0 practices. The effect of transition practices was partially mediated by an intervening effect on parent-initiated involvement in school during the kindergarten year. The findings support education policies to target kindergarten transition efforts to increase parent involvement in low-SES families.