This brief compiles the best available data on the characteristics of vulnerable young children in four service systems: Early Head Start (EHS); the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC); the child welfare (CW) system; and Part C Early Intervention Programs (Part C). Data reveal that the children and families in these systems look fairly similar on some key dimensions, suggesting that policy initiatives to support young children’s development might be informed by distilling common lessons from the systems' different research bases.