Gases and odors are generated from livestock and poultry manure decomposition shortly after it is produced, during storage and treatment and during land application. Odors originating from livestock operations may contain many different chemical compounds. Odors are characterized by concentration intensity, persistence, hedonic tone and character. Odor concentrations are measured by an olfactometer and a panel of trained assessors and usually reported as odor units. Atmospheric dispersion models are used to model odors from livestock operations. Most of the models that have been used are Gaussian plume dispersion models and include ISCST3, AUSPLUME, AERMOD, STINK, AODN, INPUFF2 and CALPUFF. A number of different modeling approaches have been used to estimate ammonia emissions from livestock operations. The emission models can be classified as mechanistic models, empirical models, mass balance models, inverse dispersion models and process based models. © 2007 ASCE.