The paper addresses flue gas cleaning processes currently used commercially in waste combustion facilities. It also discusses the operating concepts of dry, semi-dry, and wet processes, and their effectiveness in controlling various pollutants. Air pollutants from the combustion of hazardous and municipal wastes which may require control include acid gases, organic compounds, heavy metals, and particulate matter. Dry, semi-dry, and wet processes are available to control these classes of gaseous pollutants. The process that represents the optimum solution depends on many factors, including type of waste/fuel, targeted pollutants and the desired level of control for each, location of combustion unit, and residue disposal restrictions. The combustion of hazardous and municipal wastes can effectively destroy targeted substances and reduce waste volume, thus extending the useful life of existing landfills. In addition, combustion may serve the beneficial conversion of waste to energy, such as for heating, cooling, or electrical energy applications. These benefits, however, are accompanied by the emission of air pollutants that require post-combustion controls in order to meet regulatory requirements.