Scholars have long looked to family composition to understand child well-being. The study of family instability, or the experience of repeated changes in parents' union status during childhood, represents a recent advance in this field that takes into account the dynamic nature of contemporary family organization and considers its implications for children's adjustment and development. We review some of the structural and cultural factors that have contributed to rising levels of family instability and highlight the emergence of national data to measure it. We then review the perspective that guides much of the scholarship on family instability and critically assess the contributions of this work to the understanding of child well-being. We close by suggesting new directions for research, with a call for work that broadens the conceptualization and measurement of contemporary children's family systems and home environments as well as the mechanisms that explain why-or whether-instability matters. Expected final online publication date for the Annual Review of Sociology Volume 45 is July 30, 2019. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.