Children thrive in stable and nurturing environments where they have a routine and know what to expect. But a large number of children face instability at some point in their lives. They experience change in individual or family circumstances that is abrupt, involuntary, and/or in a negative direction. This paper reviews and synthesizes research on five areas of instability: family income, parental employment, family structure, housing and the out-of-home contexts of school and child care. It also discusses some of the key pathways through which instability may affect development and recommendations for policy and practice to alleviate instability’s impact.