Little is known about the effects of placing children who are abused or neglected into foster care. This paper uses the placement tendency of child protection investigators as an instrumental variable to identify causal effects of foster care on long-term outcomes--including juvenile delinquency, teen motherhood, and employment--among children in Illinois where a rotational assignment process effectively randomizes families to investigators. Large marginal treatment effect estimates suggest caution in the interpretation, but the results suggest that children on the margin of placement tend to have better outcomes when they remain at home, especially older children.