There is a dearth of research on Tier 1 instruction designed to improve the mathematics achievement of English learners. This study examined the impact of a core kindergarten mathematics curriculum on the mathematics achievement of Spanish-speaking English learners (SS-ELs). Secondary aims tested for differential response to the curriculum among SS-ELs as a function of (a) mathematics skills at the beginning of kindergarten, (b) the number of SS-ELs in classrooms, and (c) the frequency of mathematical discourse during core mathematics instruction. Data analyzed in the study were generated from a recent large-scale efficacy trial. Participants were 556 SS-ELs from 66 kindergarten classrooms. Results suggest SS-ELs in treatment classrooms made greater gains than SS-ELs in comparison classrooms on mathematics measures across the school year. Evidence of differential response to the curriculum among SS-ELs was not found. The importance of core mathematics instruction and implications for school psychologists are discussed. [This paper was published in "School Psychology Review" v45 p343-361 2016. The published article is titled "Effects of a Core Kindergarten Mathematics Curriculum on the Mathematics Achievement of Spanish-Speaking English Learners."]