In 2010, Governor Arnold Schwarzenegger signed the Kindergarten Readiness Act into law, which changed the kindergarten entry age so that children must turn 5 by September instead of December to enroll. The new grade level was put into place to promote school readiness for California's youngest learners. It also established a new grade level--transitional kindergarten (TK)--which is the first year of a two-year kindergarten experience for students born between September 2 and December 2. To address the questions and the complexities of the implementation of a statewide policy initiative, American Institutes for Research (AIR) conducted a mixed-methods study examining these issues at multiple levels of the system. Data collection strategies included surveys of district administrators (both a short-form census survey and a longer survey for a sample of districts), principals, and TK and kindergarten teachers; classroom observations; case study interviews; and parent focus groups. This executive summary highlights key findings from the study. [For the full report, "Study of California's Transitional Kindergarten Program: Report on the First Year of Implementation," see ED609102.]