The U.S. Environmental Protection Agency’s (EPA) proposed greenhouse gas (GHG) performance standards for power plants are an important step forward in regulating GHGs in terms of both their substantive impact and legal precedent. Nevertheless, we have some concerns with the proposal, which we discuss in the following comments submitted to the agency. The majority of our comments are directed to ways that EPA can increase certainty for the industry—reducing costs and, possibly, improving environmental outcomes. We highlight two specific areas of concern. First, the current proposal contributes to the significant uncertainty facing existing sources. Second, EPA’s proposed averaging option for new facilities that will install carbon capture-and-storage (CCS) technology in the future, although intended to create a flexible pathway, unfortunately creates some new regulatory uncertainty. We also comment on EPA’s decision to combine most coal and gas generators into a single source category. We believe this decision is legally valid and practically important, and that EPA should resist pressure to reconsider.