Is early preventive intervention effective in enhancing parental sensitivity and infant attachment security, and if so, what type of intervention is most successful? Seventy studies were traced, producing 88 intervention effects on sensitivity (n = 7,636) and/or attachment (n = 1,503). Randomized interventions appeared rather effective in changing insensitive parenting (d = 0.33) and infant attachment insecurity (d = 0.20). The most effective interventions used a moderate number of sessions and a clear-cut behavioral focus in families with, as well as without, multiple problems. Interventions that were more effective in enhancing parental sensitivity were also more effective in enhancing attachment security, which supports the notion of a causal role of sensitivity in shaping attachment.