The kindergarten year marks a period of rapid change in the ways children think about themselves and the world around them (Bredekamp and Copple 1997; Sameroff and McDonough 1994). Across this first year of schooling, children will acquire the knowledge and skills that will prove integral to their future success in school and in life. The kindergarten year serves multiple purposes and is geared toward the development of both cognitive and noncognitive knowledge and skills (Seefeldt 1990). And, depending on the child, knowledge and skills develop in different areas and at different rates across this year of school. To enrich the picture of children's first experience in formal education--the kindergarten year--there is a need to understand the knowledge and skills children possess as they enter kindergarten and to gain insight into how these develop across the kindergarten year. This report attempts to answer two basic sets of questions about children's knowledge and skill acquisition during the kindergarten year: (1) What gains are children making from the fall of their kindergarten year to the spring of their kindergarten year in their reading and mathematics knowledge and skills? Do these gains differ by child, family, and kindergarten program characteristics? As children are exiting kindergarten and preparing for first grade, how do their knowledge and skills differ by child, family, and kindergarten program characteristics (e.g., age, family risk factors)? and (2) What gains are children making in specific knowledge and skills (e.g., recognizing letters, recognizing numbers, paying attention)? Do children's gains in specific knowledge and skills differ by child, family, and kindergarten program characteristics? At the end of their kindergarten year when children are preparing for first grade, do their specific knowledge and skills differ by child, family, and kindergarten program characteristics (e.g., age, mother's education)? The findings in this report come from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K). The ECLS-K, sponsored by the U.S. Department of Education, National Center for Education Statistics (NCES), selected a nationally representative sample of kindergartners in the fall of 1998 and is following these children through the end of fifth grade. The full ECLS-K sample is comprised of approximately 22,000 children who attended about 1,000 kindergarten programs during the 1998-99 school year. The children attended both public (85 percent) and private (15 percent) kindergartens that offered full-day (55 percent) and part-day (45 percent) programs. All kindergarten children within the sampled schools were eligible for the sampling process, including language minority and special education students. The sample includes children from different racial/ethnic and socioeconomic backgrounds. In the fall of 1998, about 95 percent of kindergartners were entering school for the first time. This report focuses on these first-time kindergartners. An appendix contains: (1) Table 2a: Standard errors for first-time kindergartners' reading and mathematics mean by child, family, and kindergarten program characteristics: Fall 1998 to spring 1999; and (2) Table 3a: Standard errors for percentage of first-time kindergartners demonstrating specific reading knowledge and skills, by child, family, and kindergarten program characteristics: Fall 1998 to spring 1999. [Westat, Incorporated--in affiliation with the Institute for Social Research and the School of Education at the University of Michigan, and the Educational Testing Service, under the direction of the National Center for Education Statistics (NCES)--conducted the base-year study.]