This chapter reviews the growing literature on relations between sleep and externalizing and internalizing symptoms in school-age children. Included are studies that have examined a wide range of nonclinical sleep problems involving shorter duration, less optimal quality, and less consistent sleep schedules and their association with internalizing and externalizing symptoms, also within the nonclinical range. The primary goal of this chapter is to summarize and provide a synthesis of the extant literature that documents associations between sleep and adjustment. A secondary goal is to provide an overview of various conceptualizations and supportive research that may help explain the link or moderate associations between sleep and internalizing and externalizing symptoms. Most notably, the possible mediating role of emotion regulation is highlighted. Also considered are vulnerability and protective factors (e.g., individual differences) that can exacerbate or ameliorate adjustment problems in the context of poor sleep.