The Third International Mathematics and Science Study is the largest and most ambitious study ever undertaken by the International Association for the Evaluation of Educational Achievement. Forty-five countries collected data in more than 30 languages. Five grade levels were tested in the two subject areas. This report describes the science achievement of seventh and eighth graders, emphasizing the results from the eighth-grade assessment. Results are presented for the 41 countries that completed all the steps necessary to appear in this report. Singapore was the top performing country at both grade levels, with Colombia, Kuwait, and South Africa performing at the lowest levels. Perhaps the most striking finding was the large difference in average achievement between the top-performing and bottom-performing countries. Results provided a chain of overlapping countries, with most countries having an average achievement similar to that of a cluster of others, but with large differences between the top and bottom of the chain. In most countries and internationally, boys outperformed girls at both grade levels. The majority of eighth graders in nearly every country indicated that they liked science, but not all students had positive feelings about the subject. Home factors were strongly related to achievement in every participating country, but relationships between science achievement and instructional practices were less clear within and across countries. Six appendixes present information on study methodology and selected achievement results for some countries. (Contains 61 tables, 23 appendix tables, 19 figures, and 1 appendix figure.) (SLD)