This study assessed relations among placement instability, inhibitory control, and caregiver-rated child behavior. The sample included 33 adopted children who had experienced placement instability, 42 adopted children who had experienced 1 stable placement, and 27 children never placed in foster care. Five- and 6-year-old children completed the day-night task, which requires children to inhibit a prepotent response, and a control task that presents similar memory demands but does not require inhibition (C. L. Gerstadt, Y. J. Hong, & A. Diamond, 1994). Adopted children who had experienced placement instability performed worse on the inhibition task than did both other groups of children, when the authors controlled for age, verbal intelligence (as measured with the Wechsler Preschool and Primary Scale of Intelligence--Revised; D. Wechsler, 1989; or the Peabody Picture Vocabulary Test--Third Edition; L. M. Dunn & L. M. Dunn, 1997), and control task performance (p[less than]0.01). Children who had experienced placement instability were also rated on the Child Behavior Checklist as more oppositional than other children (p[less than]0.01; T. M. Achenbach & L. A. Rescorla, 2000). Inhibitory control did not mediate the association between placement instability and oppositional behavior (p[greater than]0.05). These results suggest that placement instability may adversely affect the social-emotional development of adopted children.