Foster care children who experience placement disruption and foster care instability are at elevated risk for a host of poor outcomes, yet little work considers what these unstable foster care careers look like or what causes them. In this article, I start by using previous studies on foster care drift, instability, and placement disruptions to define the unstable foster care career as a subset of foster care careers. I then use administrative data on 30,239 Danish children born 1982-1987 who entered foster care to generate nine foster care careers, two of which meet the criteria for an unstable career. Children with a high number of risk factors associated with foster care entry were also the most likely to enter an unstable career. I end by discussing implications for recent studies of the effect of foster care on children's later life outcomes and the relevance of the findings for practitioners.Keywords: administrative data; foster care; instability; sequence analysis 3 Foster care is intended to provide children with the best possible opportunity for personal development and stability when their immediate family is unable to secure them a safe and stable upbringing. Nevertheless, some children's placement trajectories end up mirroring the instability they experienced prior to entering foster care (e.g., Andersen, 2012; Havlicek 2011). Previous studies have examined aspects of such unstable foster care trajectories, including foster care drift where children move aimlessly through the foster care system, without reaching a permanent placement, adoption, or successful reunification (e.g., Andersen, 2012;Hartley, 1984)