Using the Early Childhood Longitudinal Study-Birth Cohort (n ≈ 6,800), we examined the factors explaining variation in school readiness in a large and nationally representative sample of children in immigrant and non-immigrant families. In OLS regression models with rich controls to account for selection, we found that language background was a key factor in explaining children of immigrants' expressive language and early reading at kindergarten, whereas both socioeconomic status and language background helped explain their performance in math.