This report documents the design, development, and psychometric characteristics of the assessment instruments used in the Early Childhood Longitudinal Study-Kindergarten Class of 1998-99 (ECLS-K). The ECLS-K is sponsored by the U.S. Department of Education, National Center for Education Statistics. The ECLS-K was designed to assess the relationship between a child's academic and social development and a wide range of family, school, and community variables. While the ECLS-K will ultimately span kindergarten through fifth grade, this report documents the psychometric results for four time points--fall- and spring-kindergarten and fall- and spring-first grade. Three domains are represented by the ECLS-K kindergarten and first grade (K-1) assessment instruments: cognitive (direct and indirect), socioemotional, and psychomotor. Direct cognitive measures refer to scores based on children's "direct" responses to cognitive test items. In kindergarten and first grade, direct cognitive tests were administered in reading, mathematics, and general knowledge. Indirect cognitive measures were ratings by teachers of the children's cognitive performance in closely related areas: language and literacy, mathematical thinking, and general knowledge. Following an introduction in chapter 1, chapter 2 of this report describes the objectives and design of the assessment instruments. For the cognitive tests, this includes selection of content domains, and for the direct cognitive tests, the rationale for individually administered adaptive tests, the source, and development of frameworks. Chapter 3 describes the development and field testing of the item pools for the direct cognitive measures, the selection of test items for the final forms, and the creation of a Spanish-language version of the mathematics assessment. It also introduces the criterion-referenced subsets of items selected for the reading and mathematics tests. Chapter 4 contains an overview of the Item Response Theory (IRT) procedures used to scale the test scores. Chapter 5 presents the psychometric characteristics of the direct cognitive tests given in kindergarten and first grade. Chapter 6 describes the development and psychometric characteristics of the indirect and psychomotor measures and discusses the relationship between the direct and indirect measures of the cognitive domains. Chapter 7 describes performance on the Oral Language Development Scale (OLDS), the instrument used to evaluate children's fluency in English and Spanish. Chapter 8 presents issues in analyzing longitudinal measures of cognitive skills. [The following authors contributed to this report: Sally Atkins-Burnett, Tom Hoffer, Samuel J. Meisels, Karen Tourangeau, Jerry West, and Elvira Germino Hausken.]