This special issue includes seven studies of the influence of fathers on children's development and well-being in at-risk populations. Recent research has shown that although children in these populations are at risk for various problems, families living in difficult circumstances, through poverty or other adversity, vary widely in the developmental and psychological support they provide for their children. Research reported in this issue was conducted in three different countries and shows that from infancy through middle childhood, fathers are a critical influence on family well-being and children's development in adverse environments.