The U.S. Environmental Protection Agency (EPA) proposed in 2012 proposed performance standards for carbon dioxide (CO2) emissions from new fossil fuel–fired power plants. Once finalized, the new-source standards will trigger section 111(d) of the Clean Air Act, which required the EPA to regulate CO2 emissions from existing power plants. Broad statutory language and limited legal precedent suggest that a variety of policy design options are available to the EPA and states when regulating CO2 emissions from existing power plants. At the same time, section 111(d) raises unanswered questions. In October 2012, the Nicholas Institute for Environmental Policy Solutions convened a stakeholder workshop in Washington, D.C., to discuss these questions. This report preserves the workshop discussion by summarizing panel presentations, highlighting points of conversation, and capturing key themes. This report also identifies tradeoffs facing regulators who will draft the existing-source regulations and notes issues ripe for further exploration.