The dependability of externalizing behavior composites and subscale scores from the Behavior Assessment System for Children, Second Edition, Teacher Rating Scale-Child (Reynolds & Kamphaus, 2004) and the Achenbach System of Empirically Based Assessment, Teacher's Report Form for Ages 6-18 (Achenbach & Rescorla, 2001) was investigated. Teacher pairs from six classrooms each completed items contributing to the externalizing composites and subscales for 10 of their students on two occasions approximately 1-3 weeks apart. Pearson correlation coefficients examining consistency between raters, between instruments, and between measurement occasions were generally strong, but dependability coefficients were moderate when rater, instrument, and occasion were considered concurrently. Variance component estimates indicated that individual differences among students accounted for the largest proportion of total variance across all score comparisons, but differences between teacher ratings for particular students and between instruments accounted for relatively large proportions of error variance among subscale scores. The implications of simultaneously considering multiple error sources for test users are discussed. (Contains 4 tables and 4 footnotes.)