This study investigated informant agreement on emotional and behavior problems and social skills in youth with autism spectrum disorder or intellectual disability using meta-analytic methods. Forty-nine studies were included, consisting of 107 effect sizes. The mean weighted effect size across all raters and all behaviors was 0.36, reflecting moderate agreement. Consistent with meta-analyses in typically developing youth, pairs of similar informants (e.g., parent-parent) demonstrated higher agreement compared to pairs of different raters (e.g., parent-teacher). With all rater pairs combined, agreement was significantly higher for externalizing problems (r-bar = 0.42) than either internalizing problems (r-bar = 0.35) or social skills r-bar = 0.30). Several factors appear to moderate the level of agreement among informants, including the youth's diagnosis, age, and IQ.