California's Kindergarten Readiness Act of 2010 established transitional kindergarten (TK), the first year of a two-year kindergarten program for students affected by the change in the birthdate cutoff for entry into kindergarten from December 2 to September 1. This final report from the program's evaluation focuses on results from a five-year study. The study used a rigorous regression discontinuity design to examine whether TK, as a new approach to prekindergarten education for age-eligible students, leads to positive outcomes, for which students, and under what conditions. The findings are based on direct assessments of over 6,000 kindergarteners in California in the fall and spring of the 2014-15 and 2015-16 school years, surveys of their teachers, and statewide data on students' English language proficiency. Key findings include: (1) TK gives students an advantage at kindergarten entry on a range of literacy and mathematics skills; (2) TK is effective for all groups of students who participated; (3) TK has no detectable impact on students' executive function or incidence of problem behaviors at kindergarten entry; (4) Impacts of TK are smaller at the end of kindergarten, though TK students continue to have an advantage over non-TK students on letter and word identification skills; and (5) There was little variation in the impact of TK by classroom or instructional characteristics. [For the research brief, see ED609086.]