This study provides evidence regarding the relatedness of multi-informant agreement and perceived observability of internalizing and externalizing problem behaviors. Based on 2 distinct but comparable samples of children/adolescents (Sample 1: 58.2% female and mean age of 12.51 years; Sample 2: 56.4% female and mean age of 12.35 years) and their mothers, cross-sectional as well as longitudinal analyses were conducted. Although often suggested in the literature, the results of the study do not indicate that—in general—internalizing behavior is less observable than externalizing behavior. Moreover, the results do not corroborate the suggestion that multi-informant agreement regarding internalizing behavior is lower than that of externalizing behavior. However, apart from the broad distinction of internalizing versus externalizing behavior, observability and multi-informant agreement are positively correlated with respect to reports on actual state (cross-sectional) as well as change (longitudinal) of problem behavior. The results are discussed in the context of research on multi-informant reports on problem behavior and with respect to practical implications for the assessment of problem behavior.