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 brief, the sixth in a series highlighting findings from the "Study of California's Transitional Kindergarten Program," summarizes what researchers have learned across the study about the impact of TK on students' school readiness skills, whether impacts vary for different student groups, whether those impacts are sustained, and which TK classroom characteristics are most supportive of positive outcomes. [For the full report, see ED609085.]