Purpose: The purpose of this quantitative, causal-comparative study was to determine whether English learners who participated in transitional kindergarten perform equally or better than English learners who did not participate in transitional kindergarten in the areas of oral language, written language, and overall language as measured by the Summative English Language Proficiency Assessments for California (ELPAC), in one district in Southern California. Methodology: A quantitative, ex post facto research design was used to analyze the oral, written, and overall scaled scores from the Summative ELPAC from a single district in Southern California. The sample consisted of de-identified data for 1,624 kindergarten English learners and 621 first-grade English learners. A two-way ANOVA and t tests were used to analyze the data. Findings: After controlling for gender, kindergarten and first-grade English learners who participated in transitional kindergarten outscored kindergarten and first-grade English learners who did not participate in transitional kindergarten in all areas of the Summative ELPAC. No significant interaction was found between transitional kindergarten participation and gender in all areas of the Summative ELPAC. Results also indicated females scored significantly higher than males in all areas of the Summative ELPAC. Conclusions: English learners who participated in transitional kindergarten outperformed English learners who did not participate in the program in all three areas of the Summative ELPAC: oral language, written language, and overall language. Results coincide with past literature that transitional kindergarten is beneficial for English learners. Recommendations: Further research should include a larger sample size across California, focusing on diverse student demographics such as ethnicity and language. Reasons for significant differences between female and male outcomes should also be investigated. Additional research examining other English proficiency assessments and reclassification are needed. Finally, a longitudinal study and implementation of technology should be addressed. [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.]