This report examines differences between full-day and half-day kindergarten across the United States using ECLS-K data from schools, teachers, parents and kindergarten children. Described, are the schools, both public and private, that offer these programs and the children who attend them. The report further describes many characteristics of public school full-day and half-day kindergarten classes, including specific curriculum differences between the program types. It concludes with an examination of the cognitive gains public school children make in full-day and half-day classes during the kindergarten year. Appended are: (1) Tables of Estimates; (2) Supplemental Figures: Reading and Mathematics Activities and Skills; (3) Standard Error Tables; (4) Methodology and Technical Notes. (Contains 64 tables and 52 figures.)