Common disorders of children and adolescents include neurodevelopmental disorders (e.g., intellectual disability, autistic spectrum disorder, and learning disorders), internalizing disorders (e.g., mood and anxiety disorders), and externalizing disorders (e.g., oppositional defiant disorder and conduct disorder). The assessment of a child or adolescent patient always includes multiple informants, the context in which the child’s difficulties occur, and a functional behavioral assessment. Patients with autism spectrum disorder tend to have persistent deficits in social communication and social interaction, a restricted repertoire of behaviors and interests, and abnormal cognitive functioning. Children with disruptive mood dysregulation disorder experience chronic and severe irritability and frequent temper outbursts. Attention deficit hyperactivity disorder is characterized by hyperactivity, impulsivity, and inattention before 12 years of age. Behavior therapy has been effectively used to treat children and adolescents with neurodevelopmental disorders, attention deficit hyperactivity disorder, tic disorders, feeding and elimination disorders, and externalizing disorders. Fluoxetine is approved for treatment of depression in children and escitalopram, for adolescents. Methylphenidate and amphetamine preparations are first-line treatment for children with attention deficit hyperactivity disorder.