The Trends in International Mathematics and Science Study (TIMSS) 2015 is the sixth administration of this international comparative study since 1995 when first administered. TIMSS is used to compare over time the mathematics and science knowledge and skills of fourth- and eighth-graders. TIMSS is designed to align broadly with mathematics and science curricula in the participating countries. The results, therefore, suggest the degree to which students have learned mathematics and science concepts and skills likely to have been taught in school. In 2015, TIMSS was administered in 49 IEA member countries and 6 other education systems at grade 4, and in 38 IEA member countries and 6 other education systems at grade 8. TIMSS Advanced assesses the advanced mathematics and physics knowledge and skills of students at the end of high school who have taken courses in advanced mathematics and physics. TIMSS Advanced 2015 represents only the second administration in which the United States has participated since the first administration in 1995, and is designed to align broadly with the advanced mathematics and physics curricula in the participating countries. The results, therefore, suggest the degree to which students have learned the advanced mathematics and physics concepts and skills likely to have been taught in school. Nine countries participated in TIMSS Advanced 2015. The focus of the report is on the performance of U.S. students relative to their peers in other countries on TIMSS and TIMSS Advanced 2015, and, for TIMSS results, on changes in achievement since 2011 and 1995. For a number of participating countries and education systems, changes in achievement can be documented over the last 20 years, from 1995 to 2015. This report also describes the characteristics of students who participated in the advanced mathematics and physics assessments at the end of high school, and describes the performance of males and females in these subjects. In addition, it includes achievement in Florida, a U.S. state that participated in TIMSS both as part of the U.S. national sample of public and private schools as well as individually with state-level samples of public schools. In addition to numerical scale results, TIMSS also includes international benchmarks. The TIMSS international benchmarks provide a way to interpret the scale scores by describing the types of knowledge and skills students demonstrate at different levels along the TIMSS scale. Brief Technical Notes are provided in the appendix. [For the full 2011 report, "Highlights from TIMSS 2011: Mathematics and Science Achievement of U.S. Fourth- and Eighth-Grade Students in an International Context. NCES 2013-009," see ED537756. For the supporting document, "Highlights from TIMSS 2011: Mathematics and Science Achievement of U.S. Fourth- and Eighth-Grade Students in an International Context. Appendix E: Standard Error Tables. NCES 2013-009," see ED537757.]