The aims of this investigation were to determine whether Strange Situation attachment classifications were equally valid for infants with and without extensive child-care experience in the first year of life and whether early Child Care experience, alone or in combination with mother/child factors, was associated with attachment security, and specifically with insecure-avoidant attachment. Participants were 1,153 infants and their mothers at the 10 sites of the NICHD Study of Early Child Care. Mother were interviewed, given questionnairies, and observed in play and in the home when their infants were from 1 to 15 months of age; infants were observed in child care at 6 and 15 months and in the Strange Situation at 15 months. Infants with extensive Child Care experience did not differ from infants without child-care in the distress they exhibited during separations from mother in the Strange situation or in the confidence with which trained coders assigned them attachment classifications. There were no significant main effects of Child Care experience (quality, amount, age of entry, stability, or type of care) on attachment security or avoidance. There were, however, significant man effects of maternal sensitivity and responsiveness. Significant interaction effects revealed that infants were less likely to be secure when low maternal sensitivity/responsiveness was combined with poor quality child care, more than minimal amounts of child care, or more than one care arrangement. In addition, boys experiencing many hours in care and girls in minimal amounts of care were somewhat less likely to be securely attachment.