There are relatively few empirically sound studies or nationally representative data on the number of children in Child Welfare Services (CWS) who are affected by their parents' substance abuse or dependence. The two systems that could systematically monitor this population, CWS and substance abuse treatment, are not required to capture the data elements that would identify families in both systems. The studies that are based on CWS populations or parents in treatment indicate that there is a substantial overlap in client populations. This review provides a summary of the available data; provides estimates of the overlap between populations, including the number of infants born each year with prenatal substance exposure; and suggests important opportunities to close the data gap between the systems. The findings underscore both the need for obtaining accurate data within the systems and the opportunities for states to improve their cross-system data efforts as part of their outcome monitoring.