The second National Survey of Child and Adolescent Well-Being (NSCAW II) is a longitudinal study intended to answer a range of fundamental questions about the functioning, service needs, and service use of children who come in contact with the child welfare system. The study includes 5,873 children ranging in age from birth to 17.5 years old at the time of sampling. The current analysis summarizes the well-being of these children at NSCAW II baseline. Overall, children reported for maltreatment in 2008-2009 were at higher risk for poor health and negative developmental, behavioral/emotional, and cognitive outcomes than children in the general population. Overall, 32.2 % of children from birth to 5 years old had a score indicating developmental problems. Among school-aged children and adolescents, 10.3 % showed some risk of cognitive problems or low academic achievement and 41.6 % exhibited risk of emotional or behavioral problems. Child well-being outcomes differed by age and gender but not by substantiation status or type of maltreatment. Proactively providing needed services at an early age to all children in need in the CWS is urged, because early services may well preempt these children's need for extensive future developmental, mental health, and educational services.