In this article we describe gender gaps in mathematics achievement and attitude as measured by the U.S. National Assessment of Educational Progress (NAEP) from 1990 to 2003. Analyzing relationships among achievement and mathematical content, student proficiency and percentile levels, race, and socioeconomic status (SES), we found that gender gaps favoring males (1) were generally small but had not diminished across reporting years, (2) were largest in the areas of measurement, number and operations (in Grades 8 and 12) and geometry (in Grade 12), (3) tended to be concentrated at the upper end of the score distributions, and (4) were most consistent for White, high-SES students and non-existent for Black students. In addition, we found that female students' attitudes and self-concepts related to mathematics continued to be more negative than those of male students. (Contains 6 footnotes, 3 figures, and 4 tables.)