California's Kindergarten Readiness Act of 2010 revised the cutoff date by which children must turn five for kindergarten entry in that year and established transitional kindergarten (TK), defined as the first year of a two-year kindergarten program for all students affected by the birthdate eligibility change. Since it has been implemented across the state, TK has been shown to significantly improve kindergarten readiness for California's students. But what benefits do students who might need a little extra support--such as English learner (EL) students--experience? Understanding the impact of TK on the 33 percent of California kindergartners who do not speak, read, write, or understand English well as a result of English not being their home language is critical to evaluating the success of the program. This brief, the fifth in a series highlighting findings from the "Study of California's Transitional Kindergarten Program," summarizes what we have learned about the impact of TK on EL students' school readiness skills, including mathematics skills, language and literacy skills, and English proficiency. Results demonstrated that TK has an impact on EL students' mathematics skills, language and literacy skills, and English proficiency. EL students who attend TK enjoy a particularly strong advantage over their non-TK EL peers on English proficiency. Because of this, TK may play an important role in improving academic outcomes for English learners, as students who start school with stronger academic skills tend to do better over time. Still, further examination of the potential advantages of TK participation on academic and social-emotional skills as well as English proficiency in later academic years--and the context for supporting continued growth--is needed. [For "Study of California's Transitional Kindergarten Program: Report on the First Year of Implementation," see ED609102.]