The Good Neighbor provision in Section 110(a)(2)(D) of the Clean Air Act requires each state to include measures in its State Implementation Plan (SIP) that prevent emissions within its borders from significantly contributing to nonattainment or interfering with maintenance of federal air quality standards in downwind states. When EPA determines that a state's plan is inadequate, it can impose a Federal Implementation Plan with stricter requirements. The provision exists because air pollution — especially ozone-forming nitrogen oxides from power plants and industrial sources — travels hundreds of miles across state lines, meaning downwind states can't meet clean air standards through their own actions alone.