Children need stability in their lives in order to form trusting, loving and supportive relationships. However, for a considerable number of children who enter the care system, instability is a feature of their lives. This can lead to a range of poor outcomes in both the short and the long-term. Therefore, we need to understand the factors that may contribute to stability or instability in foster care, so we can then identify what should be done in order to avoid instability as much as possible. This article is based on the author’s keynote presentation given at the Irish Foster Care Association (IFCA) conference in November 2018. It outlines an ‘ecological’ model, which refers to a holistic way of viewing human experience. Four levels of factors are identified: child factors, carer factors, social worker factors, and factors related to organisational structures. Actions to promote stability should take place on all four levels. The article draws on national and international studies. The factors and the actions/measures needed are illustrated by quotes from participants of the Care Pathways and Outcomes Study in Northern Ireland, and a study focused on stability and permanence conducted in the Republic of Ireland.