This article examines the extent of interactions or spillovers between the Supplemental Security Income (SSI) and Aid to Families with Dependent Children (AFDC) programs for children. In the early 1990s, the Social Security Administration substantially relaxed child eligibility criteria for SSI benefits. Since the changes, the number of U.S. children receiving cash and medical benefits through SSI tripled to nearly 1 million. The article describes a family's decision to participate in SSI and/or AFDC, and uses state-level data for three years before, and three years after, the Zebley decision to estimate the effect of state program generosity on child program participation. The expansions in child SSI eligibility increased child SSI participation and contributed to increased total program participation by children in the early 1990s. Child SSI participation increased more in states with lower AFDC payments and higher state SSI supplementation payments. These results suggest that families use SSI and AFDC as substitutes. At least 32 percent of the Zebley increase in SSI is likely attributable to the SSI–AFDC benefit gap for the median AFDC benefit state. © 2000 by the Association for Public Policy and Management.