English Learners (ELs) enrollment in the U.S. public schools continues to increase. ELs have various linguistic needs including listening, reading, speaking, and writing English. Often this group of students does not perform well academically on state assessments since these assessments are normally written in English and require substantial language proficiency. Further, the ELs in the United States continue to lag behind their peers who are English proficient on summative state mathematics assessment. The purpose of this study is two-fold. First, to examine if there is any relationship between English Learners' achievement on standardized mathematics assessment, specifically state summative assessment and their proficiency on the English language as measured by the English Language Proficiency Assessment for the 21st century (ELPA21). Second, the study investigated if there are any significant differences in achievement on standardized state mathematics assessment across English language proficiency classification levels (emerging, progressing, and proficient). This study employed multiple regression and one-way analysis of variance (ANOVA) to analyze archival data from an urban midwestern school district. The data was of fourth and fifth grade ELs' ELPA21 and standardized state mathematics scores from 2015 to 2017. The analysis of the data indicated a statistically significant relationship between mathematics achievement and English language proficiency among the ELs. The research findings suggest that ELs' proficiency levels in listening; reading, speaking, and writing may impact their achievement in the state summative mathematics assessment. One-way analysis of variance (ANOVA) indicated a statistically significant difference exists between the three different ELPA21 classifications of English proficiency (emerging, progressing, and proficient). English Learners who were classified as proficient on ELPA21 achieved higher scores on summative state mathematics assessment compared to those ELs who were classified as either emerging or progressing. The emerging EL group was the lowest performing. An independent samples t-test was conducted to compare the achievement scores from the state summative mathematics assessment for participants classified as progressing and proficient on ELPA21. There was a statistically significant difference in the score for emerging and proficient groups. The more proficient the EL students are on the English language, the better their performance was on the state mathematics assessments. Therefore, planning language and mathematics interventions according to the ELs' proficiency classification could help improve ELs' English language proficiency and well as their mathematics competency and impacting their achievement. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]