Forensic Outpatient Systemic Therapy (in Dutch: Forensische Ambulante Systeem Therapie; FAST) is a promising treatment for juveniles showing severe antisocial behavior, including aggression, (domestic) violence, and delinquent behavior. FAST has a flexible intensity and length, addresses individual and systemic risk and protective factors, and is responsive to the abilities of the client (system), intervention characteristics all considered crucial for effective treatment. The current study will investigate whether FAST is effective in reducing aggression of the juvenile, in reaching client formulated subgoals, and in improving family functioning (i.e., reducing juvenile-caregiver conflict and increasing caregiver responsiveness). In addition, processes of change will be examined, as well as mediation by reaching client formulated subgoals and improved family functioning. A Multiple Case Experimental Design (MCED) with an ABC design will be performed (A = baseline, B = intervention, and C = follow-up). Juveniles with primary aggression and/or anger problems (N = 15) and their caregiver(s) will be recruited.