Youth with attention deficit hyperactivity disorder experience impairment across multiple domains of functioning, with the characteristics changing with age. Thus, assessment and treatment must be appropriate for the home and school and relevant to the child’s developmental level. This chapter reviews effective assessment strategies for use with children and adolescents. Psychosocial treatments for children and adolescents are discussed separately, as the approaches with each group differ substantially. For children, strategies with a strong evidence base are described, and innovations and treatment modifications that have been examined recently are showcased. For adolescents, the results of the few randomized clinical trials conducted with this population are reviewed. A theoretical model for how to sequence treatments (i.e., intervention, medication, accommodations) for youth is referenced, and two case studies highlight this model, as well some of the new findings described in this chapter. Implications and recommendations for future research and practice are provided.