The report represents the conclusion of 4 years of analysis of large quantities of emissions, operating conditions, and boiler configuration data from full-scale multiple-burner, electric-generating boilers firing natural gas, oil, and coal fuels. The overall objective of the study was to develop from this data: (1) further understanding of the effects of combustion modifications on combustion, and the resulting effects on NOx emissions; and (2) directly applicable guidelines for the application of combustion modification techniques for the control of NOx emissions in full-scale operating utility boilers. The report includes: (1) discussion of modeling techniques used to analyze the data; (2) conclusions relative to the sources of NOx within the furnace; (3) guidelines for NOx reduction; and (4) an example application of the guidelines. Boiler firing types include single-wall, opposed and tangential configurations. The report concludes that NOx emissions are generated, in varying degrees, from conversion of fuel-bound nitrogen (the predominant source), heterogeneous combustion and mixing zone, second-stage mixing zone, and active burner region. Maintaining very fuel-rich initial combustion conditions, holding the initial peak combustion temperature to <2050 K, and delaying fuel gasification and mixing until the gas has been cooled somewhat should reduce NOx emissions from all four main sources.