Gender differences in aggression have typically been based on studies utilizing a mean difference method. From a measurement perspective, this method is inherently problematic unless an aggression measure possesses comparable validity across gender. Stated differently, establishing measurement invariance on the measure of aggression is prerequisite to making any inferences about gender differences. This paper investigated whether aggression functions in the same way across gender using teacher-rated aggression scales from the Behavior Assessment System for Children (BASC). The samples for this study consisted of the BASC standardization samples, including 1,223 children (628 boys, 595 girls) and 788 adolescents (379 boys, 409 girls). The measurement invariance of the BASC aggression scales was examined with both confirmatory factor analysis (CFA) and item response theory (IRT). CFA results showed that there was not enough evidence to support the measurement invariance of the scales across gender at a scale level; IRT results found that only a few items were significantly different across gender. The two methods have similarities and differences in studying measurement invariance, so they can be complementary to each other. The implications of the findings for research and practice in school psychology are discussed. (Contains 6 tables and 2 figures.)