The U.S. Environmental Protection Agency (EPA) is conducting a review of the air quality criteria for carbon monoxide (CO) and the national ambient air quality standards (NAAQS) for carbon monoxide (CO). The purpose of this document is to communicate the plan for this review. This review will provide an integrative assessment of relevant scientific information for CO and will focus on the basic elements of the standards: the indicator, averaging time, form, and level. These elements, which serve to define each ambient air quality standard, must be considered collectively in evaluating the degree of protection afforded by the standard. The existing primary CO standards include a 1-hour standard set at 35 parts per million (ppm), and an 8-hour standard set at 9 ppm, neither to be exceeded more than once per year. There is currently no secondary standard for CO. This review plan is organized into six chapters. Chapter 1 presents background information on the review process, the legislative requirements for the review of the NAAQS, and past reviews of the NAAQS for CO. Chapter 2 presents the current review schedule. Chapter 3 presents a set of policy-relevant questions that will serve to focus this review on the critical scientific and policy issues. Chapters 4 through 6 discuss the planned scope and organization of the key assessment documents, the planned approaches for preparing the documents, and plans for scientific and public review of the documents.