Transitional kindergarten--the first year of a two-year kindergarten program for California children born between September 2 and December 2--is intended to better prepare young five-year-olds for kindergarten and ensure a strong start to their educational career. The goal of this study was to measure the success of the program by determining the impact of transitional kindergarten on students' readiness for kindergarten in several areas. AIR researchers compared language, literacy, mathematics, executive functioning, and social-emotional skills at kindergarten entry for students who attended transitional kindergarten and for those who did not. Key findings include: (1) Transitional kindergarten improves preliteracy and literacy skills; (2) Transitional kindergarten improves students' mathematical knowledge and problem-solving skills; and (3) Transitional kindergarten supports children's behavioral self-regulation, but there is no detectable impact on social-emotional skills. This unique approach to early education in California--which serves children in a narrow age range on elementary school campuses, with credentialed teachers holding bachelor's degrees and a curriculum aligned with kindergarten--appears to better prepare students for kindergarten than what they might have received in the absence of the program. Future analyses will investigate the extent to which the transitional kindergarten advantage is sustained through the end of kindergarten, for which groups of students transitional kindergarten is most beneficial, and which transitional kindergarten program characteristics are most supportive of student learning. Detailed Methodology is appended. [For the Executive Summary of this report, see ED563820.]