The Strengthening Washington DC Families Project (SWFP) examined implementation fidelity and effectiveness when a selective, evidence-based prevention program was implemented with a sample of 715 predominantly African American families across multiple settings in an urban area. Using a true experimental design, this study reports on the differential effectiveness of four conditions (child skills training only, parent skills training only, parent and child skills training plus family skills training, and minimal treatment controls) in reducing child antisocial behavior and its precursors. Major challenges with recruitment and retention of participants and uneven program coverage were documented. No statistically significant positive effects for any of the program conditions were observed, and a statistically significant negative effect on child reports of Negative Peer Associations was observed for children of families assigned to the family skills training condition. Two marginally significant findings were observed: Child's positive adjustment favored families assigned to family skills training condition relative to minimal treatment and child training only, and family supervision and bonding was lower for children in family skills training than in the other three conditions. Hypotheses about potential explanations for the weaker than expected effects of this program are offered, as are thoughts about the infrastructure necessary to successfully implement family strengthening programs and the future of prevention science.