Clinical reasoning requires thoughtful consideration of a variety of factors that contribute to the conceptualization of a case such as the reason for referral, school information, home environment, assessment outcomes, and behavioural observations made during assessments. The purpose of this article is to provide the reader with insight into the clinical reasoning involved in the assessment and intervention planning for an adolescent with oppositional defiant disorder. The reader will be guided through the authors’ conceptualization of this case and suggestions for intervention.