Placement stability as an outcome goal in child welfare performance measurement is grounded in the importance of providing stability for children as they are developing attachments and relationships to caregivers. Research shows that many children are vulnerable to placement instability, especially those who have been in long term foster care. This literature review provides an overview of the federal placement stability measure. It then summarizes the diverse set of factors has been found to be associated with placement instability, including characteristics of the child and family of origin, placement type and quality, and the child welfare system and services. Promising practices aimed at promoting placement stability are summarized, followed by questions designed to foster discussion about the relationship of the evidence to child welfare practice.