The key issue for building an inventory is the estimation of emissions. Source emissions are estimated using a variety of approaches. Mobile sources are estimated using emission models based on vehicle testing. Point source emissions are based on stack tests or in-stack monitoring and emission factors. Area sources are almost always based on emission factors. Most air quality management programs require that the larger point sources be tested at least annually or that they operate a continuous monitor that measures emissions coming through the stack. Emission factors are based on testing carried out at similar facilities. Clearly, direct emission testing or in-stack continuous monitors can provide the most accurate emissions information for a specific source. Unfortunately, this is not possible for many sources in a region and thus emission factors must be relied upon. Chapter 6 is devoted to the discussion of building an emissions inventory.