The distinguishing and overlapping features of attention-deficit hyperactivity disorder (ADHD) and conduct disorder (CD) are discussed. Conclusions regarding comorbidity, treatment efficacy, and long-term outcome can be influenced by several factors, including diagnostic procedures and sample characteristics. The need to distinguish between referred and non-referred samples is particularly crucial when considering treatment and comorbidity issues. The efficacy of psychosocial and pharmacological treatments in ADHD and CD children is reviewed as are the few studies of psychostimulant medication in co-morbid youngsters. Suggestions regarding treatment planning and recommendations for treatment and research are described.