The Trends in International Mathematics and Science Study (TIMSS) 2003 is the third in a continuing cycle of international mathematics and science assessments conducted every four years. TIMSS assesses achievement in countries around the world and collects a rich array of information about the educational contexts for learning mathematics and science, with TIMSS 2003 involving more than 50 participants. This report contains the science results for 46 countries and four benchmarking participants at the eighth grade and for 25 countries and three benchmarking participants at the fourth grade. Trend data are provided at the eighth and fourth grades for those countries that also participated in 1995 and 1999. Following an Executive Summary and an Introduction, this book contains these eight chapters: (1) International Student Achievement in Science; (2) Performance at International Benchmarks; (3) Average Achievement in the Science Content areas; (4) Students' Backgrounds and Attitudes Towards Science; (5) The Science Curriculum; (6) Teachers of Science; (7) Classroom Characteristics and Instruction; and (8) School Contexts for Learning and Instruction. Appendices include: (1) Overview of TIMSS Procedures for Assessing Science; (2) Multiple Comparisons of Average Achievement in Science Content Areas; (3) The Text-Curriculum Matching Analysis: Science; (4) Percentiles and Standard Deviations of Science Achievement; (5) Descriptions of Science Items at Each Benchmark; (6) Syrian Arab Republic and Yemen - Science Achievement; and (7) Acknowledgements.