In recent years, success in family foster care has been evaluated by examining indicators of two goals of the child welfare system: permanency and safety. Systematic measures of child well-being have not been incorporated into the administrative databases that are used for case monitoring and evaluation. This article, a chapter in the book Family Foster Care in the Next Century, describes how child well-being has been conceptualized and measured in research on family foster care, and discusses the essential dimensions that should be included in a useful measure of child well-being. Challenges in incorporating measures of child well-being into ongoing evaluations of family foster care are discussed.