Parent-teacher cross-informant agreement, although usually modest, may provide important clinical information. Using data for 27,962 children from 21 societies, we asked the following: (a) Do parents report more problems than teachers, and does this vary by society, age, gender, or type of problem? (b) Does parent-teacher agreement vary across different problem scales or across societies? (c) How well do parents and teachers in different societies agree on problem item ratings? (d) How much do parent-teacher dyads in different societies vary in within-dyad agreement on problem items? (e) How well do parents and teachers in 21 societies agree on whether the child's problem level exceeds a deviance threshold? We used five methods to test agreement for Child Behavior Checklist (CBCL) and Teacher's Report Form (TRF) ratings. CBCL scores were higher than TRF scores on most scales, but the informant differences varied in magnitude across the societies studied. Cross-informant correlations for problem scale scores varied moderately across societies studied and were significantly higher for Externalizing than Internalizing problems. Parents and teachers tended to rate the same items as low, medium, or high, but within-dyad item agreement varied widely in every society studied. In all societies studied, both parental noncorroboration of teacher-reported deviance and teacher noncorroboration of parent-reported deviance were common. Our findings underscore the importance of obtaining information from parents and teachers when evaluating and treating children, highlight the need to use multiple methods of quantifying cross-informant agreement, and provide comprehensive baselines for patterns of parent-teacher agreement across 21 societies.