Two decisions of the U.S. Supreme Court have created enormous confusion around the question of what U.S. waters are subject to federal regulation. On May 2, EPA and the U.S. Army Corps of Engineers published proposed joint guidance that intends to clarify this issue by describing how the agencies will identify waters protected by the CWA. The document is intended to assist agency staff in implementing the Supreme Court’s rulings in Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers (SWANCC) and Rapanos v. United States. The new guidance, which would supersede existing guidance documents (including 2008 Bush Administration guidance), seeks to reaffirm jurisdiction over important waters that currently lack clear protection under the law, and to provide clearer, more predictable guidelines to reduce uncertainty and delay for businesses and regulators. Although the guidance lacks the force of regulation, it is expected to be the first step in a formal rulemaking process. On June 28, 2011, ELI convened a panel to discuss the significant elements of the guidance, assess its likely impact, and highlight the challenges that lay ahead.