Combustion turbine and gas turbine/steam combined-cycle plants have to satisfy requirements for high combustion efficiency and low pollutant emissions, in order to meet environmental regulations. Modification of the combustion process is the most cost-effective way of reducing NOx, HC, and CO emissions. The processes of NOx formation and destruction, and CO oxidation, are reviewed with reference to advanced classes of gas-turbine combustors: Ultra Lean Premix, and Topping Combustors in directly and indirectly coal-fired combined cycles. Engineering solutions to the problem of low-NOx combustion are discussed, with particular attention to the effects on combustor design of fuel type and the nature of the combined cycle.