Agreement among informants' ratings of children's and adults' internalizing and externalizing problems is moderate. Each informant contributes unique information about an individual's problems. Thus, it has been advocated to obtain ratings from multiple sources in child psychiatry, whereas adult psychiatry relies mostly on self-reports. Longitudinal studies repeatedly assessing children's psychiatric problems from childhood into adulthood and including reports from multiple informants could serve as benchmarks for studies including only selected time points or informants. We examined the development of agreement among informants' ratings of internalizing and externalizing problems using self-, parent, teacher, and partner reports in a longitudinal study with 7 assessment waves spanning an interval of 24 years and covering an age range of 4 to 40 years. The number of informant pairs is 12,059, who rated 1,875 individuals. The results revealed that correlations among informant ratings of internalizing and externalizing problems depend more on the informant pair than on problem type or age group. Second, differences among informants rating internalizing problems typically become larger when individuals get older. Third, when rating themselves, individuals typically report higher scores than do parents, teachers, or partners. These results were consistent for internalizing and externalizing problems and across age groups. The findings indicate that like in child psychiatry, assessment in adult psychiatry may benefit from a shift to multiple informant reports, as different informants' ratings may contain more information than if informants completely agree. (Contains 2 figures and 3 tables.)