This paper presents single-subject analyses of newly developing attachment relationships in 10 foster infant–caregiver dyads. Using a diary methodology, at least 2 months of daily data were provided by foster parents on infants' attachment behaviors. Foster infant attachment was also assessed using the Strange Situation. Foster mother state of mind regarding attachment was measured using the Adult Attachment Interview. For eight infants, diary data revealed predominant patterns of attachment behavior emerging within 2 months of placement. In most cases, diary data predicted Strange Situation classifications. Both Strange Situation and diary data indicated that the three children placed in foster care before 12 months of age with foster parents having primary or secondary autonomous states of mind were classified as having secure attachments. The five children placed after 12 months of age showed predominantly insecure attachment behavior in the diary and were classified as insecure in the Strange Situation. Contingency analyses of behavioral sequences reported in the diary revealed that foster parents tended to complement their foster childrens' attachment behaviors.