This manual provides guidance and documentation for users of the eighth-grade data of the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K). It begins with an overview of the ECLS-K study. Subsequent chapters provide details on the instruments and measures used, the sample design, weighting procedures, response rates, data collection and processing procedures, and the structure of the data file. The ECLS-K followed a nationally representative cohort of children from kindergarten into middle school. The base-year data were collected in the fall and spring of the 1998-99 school year when the sampled children were in kindergarten. A total of 21,260 kindergartners throughout the nation participated. (Contains 86 tables, 74 exhibits and 37 footnotes.)