The early childhood period is associated with profound development across cognitive, social, emotional, behavioral, and physical domains. Early childhood mental health is characterized by social-emotional competence and behavior regulation within healthy and supportive relationship contexts. However, children may demonstrate significant disruptions in social, emotional and behavioral functioning from early on, with approximately 12% of preschoolers in the general population and up to 30% in high-risk, low income samples identified as having serious behavioral difficulties. These challenges are associated with an elevated risk of future emotional, academic, and relationship problems. Specifically, children exhibiting early-onset behavioral problems are at especially high risk for life-course delinquency, substance use, violent behavior, academic failure, and depression. Although conduct problems are the most frequent reason children are referred for mental health services, young children's mental health problems remain underrecognized and undertreated.