In a guest column, Bill Jones, President of Blue Sky Modeling, talks about when air dispersion modeling may be subject to challenge in permitting. Modeling is the mathematical simulation of the transport and dispersion of air pollutants in the atmosphere. Modeling is typically conducted using air dispersion models which are computer programs that consider source characteristics, emission rates, meteorology, and topography to describe the behavior of air pollutants in the atmosphere. Models are frequently used in permitting efforts but also are applied in situations that do not involve permitting directly (e.g., litigation scenarios in which a model can help characterize pollutant impacts at a plaintiff location).