The U.S. Environmental Protection Agency (EPA) is finalizing a revision to regulations applicable to permitting of stationary sources of air pollution under the New Source Review (NSR) and title V programs in the Clean Air Act (CAA or Act). For sources in the oil and natural gas sector, this rule clarifies the meaning of the term "adjacent" that is used to determine the scope of a "stationary source" for purposes of the Prevention of Significant Deterioration (PSD) and Nonattainment NSR (NNSR) preconstruction permitting programs and the scope of a "major source" in the title V operating permit program in the onshore oil and natural gas sector. The revised definitions are based on the proximity of emitting activities and consideration of whether the activities share equipment. We believe that this clarification will provide greater certainty for the regulated community and for permitting authorities, and will result in more consistent determinations of the scope of a source in this sector. The EPA is adopting this revised definition in the regulations that apply to permits issued by the EPA and states to which the EPA has delegated federal authority to administer these programs. Other state and local permitting authorities with EPA-approved programs may also revise their permit programs to adopt this definition, but are not required to do so.