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 .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 (\( \bar{r} \) = .42) than either internalizing problems (\( \bar{r} \) = .35) or social skills (\( \bar{r} \) = .30). Several factors appear to moderate the level of agreement among informants, including the youth’s diagnosis, age, and IQ.