With the 2012-2013 school year, California established a new grade level called Transitional Kindergarten (TK), the first year of a two-year kindergarten experience for students who turn five between September 2 and December 2. When fully implemented, TK is intended to provide an additional year of early education for these children, with the goal of promoting their success in school. The Heising-Simons Foundation and The David and Lucile Packard Foundation are partnering with AIR to study the impact of TK on children’s academic and social-emotional skills in kindergarten, and how these impacts are related to program quality characteristics.