Depression is a leading cause of global disease burden and premature death, with early age at depression onset portending poorly for a host of negative outcomes. Children who experience depression are at risk of having a more severe course of illness, including a greater number of and more severe episodes, increased risk of suicide attempts, greater number of hospitalizations, more medical and psychiatric comorbidity, and greater impairment in social, cognitive, and occupational functioning, compared with onset in adolescence or adulthood. The treatment costs for childhood mental disorders in the US are estimated to be $10.9 billion annually. Thus, from a public health standpoint, it is critical to ascertain a clear understanding of the earliest emergence of depressive disorders and their prevalence throughout childhood so that supports and resources can be effectively used and targeted to those who need it most urgently.