Assessment and treatment of attention-deficit/hyperactivity disorder (ADHD) are reviewed in order to highlight the importance of examining individual differences in treatment response. It is emphasized that treatment response in children often varies as a function of the domain measured, the setting evaluated, and intensity of the treatment. Three case studies are presented to illustrate this point. The first case study is an example of a child who showed a consistent response to medication across settings and domains and treatment intensities. The second case study is an example of a child who showed differential treatment response as a function of setting and/or treatment intensity, but was consistent across domain. The third case study is an example of a child who showed a differential response to treatment as a function of domain, but was consistent across settings and treatment intensities. These case studies highlight the need for systematic, comprehensive, individualized treatments for children with ADHD.