Re-entry to foster care generally refers to circumstances in which children who have been discharged from foster care to be reunified with their family of origin, adopted, or provided kinship guardianship are returned to foster care. In the context of the federal performance measurement system, re-entry refers specifically to a return to foster care following an unsuccessful reunification. The federal Children and Family Services Review measures re-entry to foster care with a single indicator, called the permanency of reunification indicator, one of four indicators comprising the reunification composite measure. This review focuses on research related to the re-entry indicator, including the characteristics of children, caregivers and families, as well as case and child welfare services that are associated with a higher or lower risk of re-entry to foster care. Promising post-reunification services designed to prevent re-entry to foster care are described.