The Trends in International Mathematics and Science Study (TIMSS) is an international comparative study of student achievement. TIMSS 2011 represents the fifth such study since TIMSS was first conducted in 1995. Developed and implemented at the international level by the International Association for the Evaluation of Educational Achievement (IEA)--an international organization of national research institutions and governmental research agencies--TIMSS assesses the mathematics and science knowledge and skills of 4th- and 8th-graders. TIMSS is designed to align broadly with mathematics and science curricula in the participating countries and education systems. This report focuses on the performance of U.S. students relative to their peers around the world in countries and other education systems that participated in TIMSS 2011. For the purposes of this report, "countries" are complete, independent political entities, whereas "other education systems" represent a portion of a country, nation, kingdom, or emirate or are other non-national entities (e.g., U.S. states, Canadian provinces, Flemish Belgium, and Northern Ireland). In this report, these "other education systems" are designated as such by their national three-letter international abbreviation appended to their name (e.g., England-GBR, Ontario-CAN). This report also examines changes in mathematics and science achievement compared with TIMSS 1995 and TIMSS 2007. In 2011, TIMSS was administered at grade 4 in 57 countries and other education systems and, at grade 8, in 56 countries and other education systems. These total counts include U.S. states that participated in TIMSS 2011 not only as part of the U.S. national sample of public and private schools but also individually with state-level public school samples. At grade 4, this was Florida and North Carolina, and at grade 8 this was Alabama, California, Colorado, Connecticut, Florida, Indiana, Massachusetts, Minnesota, and North Carolina. Note that, because all TIMSS participants are treated equally, these states are compared with the United States (national sample) throughout this report. All differences described in this report are statistically significant at the 0.05 level. No statistical adjustments to account for multiple comparisons were used. Appended are: (1) Technical Notes; (2) Example Items; (3) TIMSS-NAEP Comparison; and (4) Online Resources and Publications. (Contains 53 tables, 16 figures, 18 exhibits and 48 footnotes.) [For 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.]