The Third International Mathematics and Science Study (TIMSS) is the most ambitious study conducted by the International Association for the Evaluation of Educational Achievement to date. TIMSS developed and administered tests and questionnaires in three student populations to study achievement in participating countries and the factors associated with high achievement. Contributions to this volume describe the main elements of the effort to assure the quality of TIMSS data, focusing on such issues as the translation of the achievement tests from English into the 30 other languages of the study, sampling from the 45 participating countries, and reliability questions. The following chapters are included: (1) "Translation, Verification" (Ina V. S. Mullis, Dana L. Kelly, and Kathleen Haley); (2) "Sampling" (Pierre Foy, Michael O. Martin, and Dana L. Kelly); (3) "Monitoring the TIMSS Data Collection" (Michael O. Martin, Craig D. Hoyle, and Kelvin D. Gregory); (4) "Observing the TIMSS Test Administration" (Michael O. Martin, Craig D. Hoyle, and Kelvin D. Gregory); (5) "Quality Control Steps for Free-Response Scoring" (Ina V. S. Mullis and Teresa A. Smith); and (6) "Data Consistency Checking across Countries" (Heiko Jungclaus and Michael Bruneforth). Nine appendixes present supplemental information about study methodology. (Contains 1 figure, 28 tables, and 3 appendix tables.) (SLD)