This paper documents the evolution of Environmental Protection Agency (EPA) policy concerning the relative roles that volatile organic compounds (VOCs) and oxides of nitrogen (NOx) emissions play in ozone nonattainment planning for State implementation plans (SIPs). It further discusses possible new control requirements, including NO x measures, that may be required as a result of pending Clean Air Act Amendments. In the early 1970's EPA guidance emphasized the use of VOC control measures to attain the ozone (formerly oxidant) ambient air quality standard. Little if any, control requirements applied to NO x emissions. EPA continued to focus guidance requirements upon the control of VOCs during the planning efforts associated with the 1977 Clean Air Act amendments... 1979 SIPs, 1982 plans for long-term problem areas (extension areas), and other revisions to SIPs. Preliminary air quality modeling work to support these later planning efforts revealed that, in certain cases, there were potential benefits of NO x control in addition to VOC. With this new insight, EPA's post-1987 policy proposal required states to look at the potential benefits of NO x controls in areas with a nonmethane organic compound to NO x ratio of 10:1 or greater. In Clean Air Act Amendments currently being debated in the House and Senate, there is an uncertainty with regard to the role of NO x controls. Possibilities under consideration range from the application of reasonably available control technology for 100 ton sources of NO x to a clean fuel motor vehicle program within 42 months of enactment.