The externalizing spectrum may explain covariation among externalizing disorders observed in childhood and adulthood. Few prospective studies have examined whether externalizing spectrum might manifest differently across time, reporters, and gender during childhood. We used a multitrait, multimethod model with parent and teacher report of attention-deficit/hyperactivity disorder (ADHD) symptoms, oppositional defiant disorder (ODD) symptoms, and conduct disorder (CD)symptoms from kindergarten to Grade 5 in data from the Fast Track Project, a large multisite trial for children at risk for conduct problems ( n = 754). The externalizing spectrum was stably related to ADHD, ODD, and CD symptoms from kindergarten to Grade 5, with similar contributions from parents and teachers. Configural, metric, and scalar invariance were largely supported across time, suggesting that the structure of the externalizing spectrum is stable over time. Configural and partial metric invariance were supported across gender, but scalar invariance was not supported, with intercepts consistently higher for males than for females. Overall, our findings confirm other research that the externalizing spectrum can be observed early in development as covariation between ADHD, ODD, and CD, and extend that work to show that it is relatively consistent across time and reporter, but not consistent across gender.