Research suggests that providing high-quality early education experiences for young learners can support positive outcomes for these children, including higher achievement test scores and even higher graduation rates. Transitional kindergarten (TK) aims to provide an additional year of kindergarten to California's youngest students to help them obtain the necessary preparation to succeed in school. In the fall of 2012, districts began the rollout of TK by moving back the kindergarten entry age by one month and offering TK to children who turned five between November 2 and December 2, 2012. Through an analysis of data from a survey of all elementary and unified districts in California, this research brief provides an initial glimpse of transitional kindergarten in its first year of statewide implementation. The study finds that the majority of districts in California implemented TK in 2012-13, serving approximately 39,000 students. While large districts were most likely to offer standalone TK classrooms, small districts with fewer students almost exclusively provided TK in combination classrooms. Future research briefs based on this continuing study will explore this variation as we describe the implementation of TK in greater depth at the district, school, and classroom levels. [For "Study of California's Transitional Kindergarten Program: Report on the First Year of Implementation," see ED609102.]