According to EPA's Superfund information system, there were 3,397 active Superfund enforcement instruments to ensure cleanups at National Priorities List sites as of September 30, 2007. Yet, EPA does not nationally compile or track data on substantial non-compliance (SNC) with the terms or requirements of these instruments. Therefore, we were not able to fully determine whether the regions have resolved Superfund instrument violations consistent with criteria and authorities. In 2000, though, EPA recognized it needed to improve in this area. It issued an internal report recommending that the regions improve their data on the compliance status of Superfund enforcement instruments and responses to noncompliance. However, EPA has not implemented this recommendation. Consequently, the Agency lacks the internal controls necessary to monitor compliance with Superfund instruments nationally. In a limited review of EPA regions enforcement records, we found that two regions enforcement actions, in 12 instances of SNC, were consistent with EPA guidance and authorities. While the regions took appropriate actions to address these 12 violations, Region 5 had not established necessary and enforceable requirements to address contamination from the Muskego Landfill Site, in Waukesha County, Wisconsin.