The U.S. Environmental Protection Agency (EPA) proposed a New Source Performance Standard (NSPS) on spark ignition (SI) stationary internal combustion engines in May 2006 and will promulgate this rule by December 20, 2007. This rule, which is in response to a settlement agreement and is under the authority of section 111(b) of the Clean Air Act, will address emissions for nitrogen oxides (NOx), particulate matter (PM), and carbon monoxide (CO) from new SI engines. The NSPS contains requirements for owners, operators, and manufacturers of stationary SI engines. By model year 2015, 411 stationary SI engines must be certified to the final Tier 4 emission standards for all pollutants. In addition, EPA proposed simultaneously a national standard to address hazardous air pollutant (NESHAP) emissions from existing and new stationary SI engines. These rules together are considered economically significant according to Executive Order 12866 because the benefits and costs together for these rules are likely to exceed $100 million. Because of the effect of a recent DC Circuit Court of Appeals decision on the legality of another NESHAP, EPA has decided not to promulgate a standard to address HAP emissions from existing stationary SI engines by December 2007. HAP emissions from those engines will be addressed in a separate rulemaking that will take place after December 20, 2007. The stationary SI NSPS and new area source NESHAP will be promulgated by December 20, 2007, as currently planned. As part of the regulatory process of preparing these standards EPA is required to develop a regulatory impact analysis (RIA). This RIA includes an economic impact analysis (EIA), a small entity impacts analysis and a benefits analysis for the final rule to be promulgated in December, 2007. This report documents the methods and results of this RIA.