Attachment security in children's first relationships with their primary caregivers in infancy supports multiple aspects of their social development during childhood. Insecure attachment, in contrast, underlies a wide range of negative outcomes for children, including aggressive behavior toward others. In a sample of 161 low-income families, early attachment insecurity was examined in relation to physical punishment and child aggression at ages two and three in the context of risk and intervention. Families had been randomly assigned either to a comparison group or to an Early Head Start program that provided weekly home visits aimed at increasing positive aspects of parenting behaviors. Children with higher security scores were less likely to be spanked at age three and less likely to be aggressive at ages two and three. Early Head Start made an independent contribution to less spanking at age three, over and above family risk factors, earlier attachment security, or earlier spanking. The results suggest that a secure attachment relationship and a parenting-focused home visiting intervention can reduce the physical punishment associated with child aggression and thereby indirectly reduce early childhood aggression.