The Early Childhood Longitudinal Study-Kindergarten Class of 1998-99 (ECLS-K), selected a nationally representative sample of approximately 22,000 kindergartners in the fall of 1998 and is following these children through the end of the fifth grade. Baseline data about these children, their families, and their kindergarten programs were collected by means of telephone interviews with the children's parents or guardians and from self-administered questionnaires completed by the kindergarten teachers. Data were also gathered during an individual assessment with each child. This report documents the design, development, and psychometric characteristics of the assessment instruments used in the ECLS-K. The focus is on the psychometric results of the assessment instruments for four time points: Fall- and Spring-kindergarten and Fall- and Spring-first grade. The assessment instrument examined three domains: the cognitive (direct and indirect), socioemotional, and psychomotor. In addition, the report discusses issues involved in analyzing longitudinal measures of cognitive skills, including the use of total scores and of proficiency probabilities to measure longitudinal change. Initial results revealed sex differences in prereading skills at kindergarten entry and the areas of gain. Public school children had the lowest reading skills at kindergarten entry, followed by Catholic school children, with private non-Catholic school children having the highest reading skills. There were differences in the areas of gain in children attending different types of schools. The report's five appendices include a summary of national mathematics and science curriculum standards, reading assessment content classifications used for test item development, ECLS item parameters and item fit by rounds, and score statistics for indirect and psychomotor measures for selected subgroups. (Contains 60 references.) (KB)