This methodological report documents the design, development, and psychometric characteristics of the assessment instruments used in the third grade data collection of the ECLS-K. The instruments examined include those developed to measure cognitive and socioemotional development. In addition, issues in analyzing longitudinal measures are discussed. This report is organized into the following chapters: (1) Introduction; (2) Design and Development of the Assessment Instruments; (3) Analysis Methodology; (4) Psychometric Characteristics of the ECLS-K Direct Cognitive Battery; (5) Direct Cognitive Assessments: Longitudinal Measurement; and (6) Psychometric Characteristics of the Indirect Measures and the Direct Self Description Questionnaire. The following are appended: (1) Score Statistics for Direct Cognitive Measures for Selected Subgroups; and (2) ECLS-K Item Parameters and Item Fit by Rounds. (Contains 100 tables, 2 exhibits, and 5 figures.)