In the context of current concern about levels of marital distress, family violence, and divorce, the SFI study is evaluating the effectiveness of an intervention to facilitate the positive involvement of low-income Mexican American and European American fathers with their children, in part by strengthening the men's relationships with their children's mothers. The study design involves a randomized clinical trial that includes assignment to a 16-week couples group, a 16-week fathers group, or a single-session control group. Couples in both group interventions and the control condition include partners who are married, cohabiting, and living separately but raising a young child together. This article presents the rationale, design, and intervention approach to father involvement for families whose relationships are at risk because of the hardships of their lives, many of whom are manifesting some degree of individual or relationship distress. We present preliminary impressions and qualitative findings based on our experience with 257 families who completed the pretest, and the first 160 who completed one postintervention assessment 9 months after entering the study. Discussion centers on what we have learned and questions that remain to be answered in mounting a multisite preventive intervention to strengthen relationships in low-income families.