The impact of California's transitional kindergarten program on Spanish-speaking dual language learners was examined through two studies. Participants in the two studies included: (1) the statewide population of students who met study inclusion criteria (n = 45,010) and took the California English Language Development Test (CELDT), and (2) a sample of students (n = 1868) in 20 school districts. Findings indicate that TK had moderate to large effects on English proficiency; smaller but statistically significant effects on language, literacy, and math skills; and no effects on social-emotional skills or executive function. The transitional kindergarten program provides participating Spanish-speaking dual language learners with an academic advantage at kindergarten entry, as compared to Spanish-speaking dual language learners who do not attend. (author abstract)