Using 3 waves of the TIMSS data, this study investigates the relationship between 8th-graders' mathematics and science achievement and their self-perceptions on these 2 subjects. For within-country data, there is generally a positive relationship between students' achievement and the following 3 measures of their self-perception: how much they like the 2 subjects, their self-perceived competence in the subjects, and their perceived easiness of the subjects. However, in a between-country analysis, the relationship is opposite. More specifically, when the same 3 self-perception measures are aggregated at the country level, there is a negative relationship between self-perceptions and achievement. These findings are consistent for both mathematics and science across the data for all 3 waves. Correlation analysis is used for both within-country and between-country analyses. Among several possible explanations, the authors suggest that this pattern may reflect high academic standards in high-performing countries and low academic standards in low-performing countries. (Contains 6 figures and 2 tables.)