In August 2007, the Centers for Medicare and Medicaid Services (CMS) issued a directive requiring that states reach participation rates of 95 percent among children in families with income below 200 percent of the federal poverty line in Medicaid and the State Childrens Health Insurance Program (SCHIP) before using SCHIP funds to cover higher-income children. In this brief, Genevieve Kenney shows that there are serious methodological challenges involved with obtaining valid and policy-relevant state-level participation rate estimates. Kenney also reviews recent trends in Medicaid and SCHIP participation rates and comments on participation estimates just released by CMS.