Technical Abstract: Air quality, especially as related to odor and airborne particulates, is an issue confronting by-product handlers, processors, and land applicators, as well as communities in which by-products, such as manure, sewage sludge, and biosolids, are generated or applied. Odor problems and their prevention and mitigation are complex and involve specialized measurement, identification, characterization, and testing techniques. This chapter describes the major groups of chemical compounds that degrade air quality, either in terms of odor, gaseous pollutants, or particulates, and that may be associated with by-product handling, processing, or land application. In addition, methods for assessing community annoyance due to odor, and for odor sampling and analysis, including the possible use of the electronic nose, are described. Finally, approaches to prevention and mitigation, including the role of stabilization, are provided.