Goodness of fit has been a key theoretical construct for understanding caregiver-child relationships. We developed an interview method to assess goodness of fit as a relationship construct, and employed this method in a longitudinal study of child temperament, family context, and attachment relationship formation. Goodness of fit at 4 and 8 months of age predicted secure attachment at 15 months, after accounting for variance associated with SES, maternal depression, child temperament, parenting quality, and family functioning. Structural models indicated that prediction of security by goodness of fit and parenting quality was relatively independent. The goodness-of-fit pathway was most associated with maternal depression, whereas the parenting sensitivity pathway was most associated with child temperament, family functioning, and SES.