This brief examines Trends in International Mathematics and Science Study (TIMSS) science achievement of the 20% highest and the 20% lowest performing fourth and eighth graders in 11 countries. To do so, the authors compare achievement distributions across countries, using measures of relative equality within countries. Such an examination provides detailed insights into variation, and has the potential to reveal tailored options for addressing various issues. For example, if gender gaps exist at the lower end of the achievement distribution, policies addressing the specific weaknesses of low achieving students of the disadvantaged gender would be more effective than any generalized measures implemented for all students. Therefore, the authors compare (1) general changes in gender gaps in science achievement at the tails of the achievement distribution over time; and (2) the development of gender gaps between grade four and grade eight within a cohort of students at two different points in time, 16 years apart.