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.