This article describes a treatment project designed to examine the effectiveness and generalization of Parent-Child Interaction Therapy (PCIT) with families of preschool-aged children with conduct problem behavior. The importance of early intervention and issues related to measurement of change in these young children and their families are discussed. The treatment program and the study design are described, with particular emphasis on the measures used to assess treatment outcome. The sensitivity of the measures to change is illustrated with data from the first few families who have completed treatment.