To determine whether California's TK program is effective at improving school readiness and learning outcomes for students, American Institutes for Research (AIR) conducted an evaluation of TK as it was implemented during the 2013-14 and 2014-15 school years. This study uses 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. Previous research has shown that participation in high-quality early education prior to kindergarten can improve young children's readiness skills for elementary school, positively affecting behavioral, social-emotional, and cognitive outcomes in particular. Specifically, the study addresses the following questions: 1. Does TK improve kindergarten readiness in the domains of early literacy/language, mathematics, and social-emotional skills? 2. How do impacts vary by student background characteristics, such as gender, English learner status, or poverty status? 3. To what extent are the impacts of TK sustained through the end of kindergarten? 4. What are the characteristics of TK classrooms, and how does the impact of the program vary with differences in these characteristics? (author abstract)