There is little research on knowledge of the policy rules that could affect individuals, either in general or in evaluations of new programs. The lack of research is surprising, given that knowledge gaps could limit the effectiveness of reforms or lead to incorrect inferences regarding the effects of a policy change. In this article, we use survey data to examine the level and sources of welfare participants' policy knowledge in the context of a substantial change in child support and welfare policy in Wisconsin. We find very low levels of knowledge of child support policy rules. Multivariate analyses suggest that people tend to learn policy rules by experience; we find less consistent support for knowledge being primarily imparted through interactions with caseworkers. A difference-in-difference analysis suggests that if participants had been more knowledgeable, program impacts would have been larger. Implications of this research for policy implementation and policy evaluations are discussed. (Contains 4 tables and 18 footnotes.)