Over the past decade, excessive sleepiness among children and adolescents has been identified as a major societal concern. Professionals working with pediatric groups must increasingly factor sleepiness into assessments of waking function. We define and discuss excessive sleepiness in children and adolescents and review available evidence regarding effects on behavior, mood, and performance. Findings for daytime sleepiness and subsequent impairment in these domains are classified as robust to unknown. Empirical evidence clearly indicates that children and adolescents experience significant daytime sleepiness as a result of inadequate or disturbed sleep. The specific effect of sleepiness on functional domains in pediatric groups are less well-studied, but existing data suggests that children are likely to experience impairment in behavioral, mood, and performance domains. However, such variables as developmental differences in the type and degree of impairment, the degree of sleep disturbance required to produce impairments, and potential risk and protective factors for the effects of sleepiness in children have yet to be described. Further research is clearly warranted, and we discuss important questions and methodological concerns to encourage inquiry in both clinical and experimental settings. Advice is offered with regard to screening for sleep problems and associated sleepiness with children and adolescents.