The paper reviews the results of theoretical and experimental study on development and application of the method aimed at excess air optimization for utility/industrial steam boilers fired with fossil fuels. A concept of the cost-based excess air optimization is presented. Various application options (experimental/theoretical; firing different fossil fuels in utility/industrial boilers; pursuing distinct goals, etc.) are discussed. Three case studies including utility boilers firing lignite and fuel oil as well as an industrial boiler installed at a refinery are reviewed. Limitations and constraints in method application are discussed as well. As shown in this work, switching the combustion excess air to the optimized or "compromise" values ensures noticeable reduction of the total operational costs, associated with the fuel consumption ("internal" costs) and environmental impact ("external" costs), for the particular boiler.