Using K-8 national longitudinal data, the authors investigate males' and females' achievement in math and reading, including when gender gaps first appear, whether the appearance of gaps depends on the metric used, and where on the achievement distribution gaps are most prevalent. Additionally, teachers' assessments of males and females are compared. The authors find no math gender gap in kindergarten, except at the top of the distribution; however, females throughout the distribution lose ground in elementary school and regain some in middle school. In reading, gaps favoring females generally narrow but widen among low-achieving students. However, teachers consistently rate females higher than males in both subjects, even when cognitive assessments suggest that males have an advantage. Implications for policy and further research are discussed. (Contains 9 notes, 2 tables and 7 figures.)