This paper addresses the role of family-based studies of preventive and therapeutic interventions in our understanding of normal development and psychopathology. The emphasis is on interventions designed to improve parent–child and/or marital relationships as a way of facilitating development and reducing psychopathology in children and adolescents. Intervention designs provide the gold standard for testing causal hypotheses. We begin by discussing the complexity of validating these hypotheses and the implications of the shift from a traditional emphasis on theories of etiology to developmental psychopathology's newer paradigm describing risks → pathways → outcomes. We summarize correlational studies that document the fact that difficult and ineffective parent–child and marital relationships function as risk factors for children's cognitive, social, and emotional problems in childhood and adolescence. We then review prevention studies and therapy evaluation studies that establish some specific parenting and marital variables as causal risk factors with respect to these outcomes. Our discussion focuses on what intervention studies have revealed so far and suggests an agenda for further research.