Title IV of the Clean Air Act (CAA or the Act), as amended by the Clean Air Act Amendments of 1990, authorizes the Environmental Protection Agency (EPA or Agency) to establish the Acid Rain Program. The Acid Rain Program and the provisions in this final rule benefit the environment by ensuring that the sulfur dioxide (SO<INF>2</INF>), nitrogen oxides (NO<INF>X</INF>) and carbon dioxide (CO<INF>2</INF>) air pollution emissions to be measured and tracked pursuant to the provisions of 40 CFR part 75 are accurately monitored and reported. These provisions also benefit the regulated entities by providing additional flexibility and improved cost effectiveness to the monitoring and reporting options available to part 75 subject sources. On January 11, 1993, the Agency promulgated final rules, including the final continuous emission monitoring (CEM) rule, under title IV. On May 17, 1995 and November 20, 1996, the Agency revised the CEM rule to make the implementation simpler. On May 21, 1998, the Agency proposed additional revisions to the CEM rule, to make implementation easier and more efficient for both EPA and the facilities affected by the rule, to improve quality assurance requirements, and to create new alternative monitoring options. EPA promulgated final rule revisions addressing some of these additional proposed revisions, based on comments received, when EPA promulgated a Finding of Significant Contribution and Rulemaking for Certain States in the Ozone Transport Assessment Group Region for Purposes of Reducing Regional Transport of Ozone (NO<INF>X</INF> SIP call). In this action, EPA is issuing final rule revisions addressing the remaining May 21, 1998 proposed revisions to the CEM rule, with certain changes to the proposal based on the public comments received. Some of these revisions will be relevant for sources that become subject to part 75 requirements in response to the NO<INF>X</INF> SIP call.