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 fifth short report in a series highlighting findings from the Study of California's Transitional Kindergarten Program focuses on what we have learned about the structure, teachers, and instruction in TK classrooms in the 2014-15 school year. The findings are based on a survey of 200 TK teachers and 184 classroom observations in 20 school districts conducted in spring 2015. Three years into the implementation of transitional kindergarten, we examine the characteristics of TK programs across the state--the structure of TK classrooms, characteristics of teachers, and the content and organization of classroom instruction. The surveys of TK teachers and observations of their classrooms conducted in spring 2015 provide a snapshot of how TK was being implemented in the third year of statewide rollout of the program. Key findings: (1) The majority of TK classrooms provided a structure that may be conducive to supporting student readiness for kindergarten; (2) The average TK classroom was smaller than kindergarten classrooms, but the ratio of teachers to students was lower than is required for State Preschool; (3) Standalone TK classrooms provide a more balanced curriculum and a more developmentally appropriate experience for TK students than TK/K combination classrooms; (4) Teachers in both standalone TK and TK/K combination classrooms demonstrated moderately high quality interactions in terms of emotional support and classroom organization; and (5) Districts could provide TK teachers more guidance on how to promote children's thinking and problem solving, use feedback to deepen understanding, and help children develop more complex language skills. [For "Impact of California's Transitional Kindergarten Program, 2013-14," see ED563818.]