EPA is in the process of regulating U.S. greenhouse gas (GHG) emissions using its powers under the Clean Air Act. The likely next phase of this regulatory program is performance standards under Section 111 of the act for coal plants and petroleum refineries, which the agency has committed to finalize by the end of 2012. Section 111 appears to allow use of flexible, market-based regulatory tools. In this paper, we discuss one such tool, tradable standards. Tradable standards appear to be a legally and politically viable choice for the agency, and evidence suggests they are substantially more cost-effective than traditional performance standards. The paper discusses implementation issues with tradable standards, including categorization, banking, and phased implementation, as well as broader issues with the Section 111 rulemaking process as it relates to state-level GHG regulatory efforts.