This Clinical Case Conference presents a patient who was admitted for his first psychiatric hospitalization at age 12 initially to an inpatient children’s unit but who was extraordinarily aggressive and difficult to treat and was eventually transferred to the adolescent inpatient unit when he turned 13 years old. The panel discusses their experiences in treating this patient including the challenges of maintaining team cohesion when treating a difficult patient who engenders extreme countertransference because of his continued episodes of aggression despite the best efforts of all involved. The “differential diagnosis” of aggressive behavior is reviewed including a discussion of the biological underpinnings of aggression in the developing child and adolescent. In addition, the evolving differential diagnoses for this patient are discussed to include specifically: posttraumatic stress disorder, ADHD, learning disabilities/cognitive deficits, expressive and receptive language disorder, cultural factors, major depressive disorder, psychotic disorders, vision impairment, epilepsy, and conduct disorder. Treatment algorithms with both psychotherapeutic and psychopharmacological approaches are reviewed.