Serious emotional disturbance (SED) in children, adolescents, and young adults is defined as a diagnosable mental, behavioral, or emotional disorder that substantially impairs a child’s functioning in family, school, or community contexts. SED is estimated to affect roughly 5 to 10 percent of youth in the United States, translating to approximately 5 to 7 million children and adolescents. SED crosses demographic and socioeconomic groups, with exposure to trauma, family history of mental illness, poverty, and access to supportive resources on par with needs contributing to some of the risk. Comorbidities such as substance use disorders, suicidality, and intellectual and developmental disabilities are prevalent and complicate diagnosis and service eligibility. This paper provides an overview of SED, including the role of functional impairment in identifying youth with SED and the important developmental considerations. Best practices in assessment and treatment approaches are presented, including a discussion of the evidence- based service arrays needed to support youth with SED and the importance of long-term maintenance strategies and policy supports to sustain functioning gains. It includes briefs for various SED diagnostic categories, which outline the clinical presentation, prevalence, risk factors, comorbidities, treatments, and recommendations for state service systems. Finally, the paper calls for policies that ensure access to services and integrate wellness and prevention strategies, such as exercise, mindfulness, and nutrition, as part of a holistic approach to youth mental health.