This Statistics in Brief (SiB) uses data from the 2011 and 2019 Trends in International Mathematics and Science Study (TIMSS), and explores how achievement gaps between high- and low-performing 4th- and 8th-grade students in the U.S. and other education systems have changed over time. Achievement gaps are defined as the differences in scores between students at the 90th percentile of performance (high-performing) and those at the 10th percentile of performance (low-performing); they are also known as "score gaps." The SiB focuses on two questions for each grade and subject: (1) In which education systems did score gaps between high- and low-performing students change (widen or narrow) between 2011 and 2019? (2) Is the widening or narrowing of score gaps between 2011 and 2019 driven primarily by changes in the scores of high-performing students, low-performing students, or both?